thodg/libgit2

diff --git a/src/config_file.c b/src/config_file.c
index 010c494..afd80d3 100644
--- a/src/config_file.c
+++ b/src/config_file.c
@@ -114,8 +114,7 @@ typedef struct {
 	diskfile_backend *snapshot_from;
 } diskfile_readonly_backend;
 
-static int config_parse(git_strmap *values, diskfile_backend *cfg_file, struct reader *reader, git_config_level_t level, int depth);
-static int parse_variable(struct reader *reader, char **var_name, char **var_value);
+static int config_read(git_strmap *values, diskfile_backend *cfg_file, struct reader *reader, git_config_level_t level, int depth);
 static int config_write(diskfile_backend *cfg, const char *key, const regex_t *preg, const char *value);
 static char *escape_value(const char *ptr);
 
@@ -288,7 +287,7 @@ static int config_open(git_config_backend *cfg, git_config_level_t level)
 	if (res == GIT_ENOTFOUND)
 		return 0;
 
-	if (res < 0 || (res = config_parse(b->header.values->values, b, reader, level, 0)) < 0) {
+	if (res < 0 || (res = config_read(b->header.values->values, b, reader, level, 0)) < 0) {
 		refcounted_strmap_free(b->header.values);
 		b->header.values = NULL;
 	}
@@ -313,7 +312,7 @@ static int config__refresh(git_config_backend *cfg)
 	reader = git_array_get(b->readers, git_array_size(b->readers) - 1);
 	GITERR_CHECK_ALLOC(reader);
 
-	if ((error = config_parse(values->values, b, reader, b->level, 0)) < 0)
+	if ((error = config_read(values->values, b, reader, b->level, 0)) < 0)
 		goto out;
 
 	git_mutex_lock(&b->header.values_mutex);
@@ -831,7 +830,7 @@ static int reader_getchar_raw(struct reader *reader)
 
 	if (c == 0) {
 		reader->eof = 1;
-		c = '\n';
+		c = '\0';
 	}
 
 	return c;
@@ -850,13 +849,12 @@ static int reader_getchar(struct reader *reader, int flags)
 
 	do {
 		c = reader_getchar_raw(reader);
-	} while (skip_whitespace && git__isspace(c) &&
-	       !reader->eof);
+	} while (c != '\n' && c != '\0' && skip_whitespace && git__isspace(c));
 
 	if (skip_comments && (c == '#' || c == ';')) {
 		do {
 			c = reader_getchar_raw(reader);
-		} while (c != '\n');
+		} while (c != '\n' && c != '\0');
 	}
 
 	return c;
@@ -1200,398 +1198,6 @@ static int included_path(git_buf *out, const char *dir, const char *path)
 	return git_path_join_unrooted(out, path, dir, NULL);
 }
 
-static int config_parse(git_strmap *values, diskfile_backend *cfg_file, struct reader *reader, git_config_level_t level, int depth)
-{
-	int c;
-	char *current_section = NULL;
-	char *var_name;
-	char *var_value;
-	cvar_t *var;
-	git_buf buf = GIT_BUF_INIT;
-	int result = 0;
-	uint32_t reader_idx;
-
-	if (depth >= MAX_INCLUDE_DEPTH) {
-		giterr_set(GITERR_CONFIG, "Maximum config include depth reached");
-		return -1;
-	}
-
-	reader_idx = git_array_size(cfg_file->readers) - 1;
-	/* Initialize the reading position */
-	reader->read_ptr = reader->buffer.ptr;
-	reader->eof = 0;
-
-	/* If the file is empty, there's nothing for us to do */
-	if (*reader->read_ptr == '\0')
-		return 0;
-
-	skip_bom(reader);
-
-	while (result == 0 && !reader->eof) {
-
-		c = reader_peek(reader, SKIP_WHITESPACE);
-
-		switch (c) {
-		case '\n': /* EOF when peeking, set EOF in the reader to exit the loop */
-			reader->eof = 1;
-			break;
-
-		case '[': /* section header, new section begins */
-			git__free(current_section);
-			current_section = NULL;
-			result = parse_section_header(reader, &current_section);
-			break;
-
-		case ';':
-		case '#':
-			reader_consume_line(reader);
-			break;
-
-		default: /* assume variable declaration */
-			result = parse_variable(reader, &var_name, &var_value);
-			if (result < 0)
-				break;
-
-			git__strtolower(var_name);
-			git_buf_printf(&buf, "%s.%s", current_section, var_name);
-			git__free(var_name);
-
-			if (git_buf_oom(&buf)) {
-				git__free(var_value);
-				return -1;
-			}
-
-			var = git__calloc(1, sizeof(cvar_t));
-			GITERR_CHECK_ALLOC(var);
-			var->entry = git__calloc(1, sizeof(git_config_entry));
-			GITERR_CHECK_ALLOC(var->entry);
-
-			var->entry->name = git_buf_detach(&buf);
-			var->entry->value = var_value;
-			var->entry->level = level;
-			var->included = !!depth;
-
-
-			if ((result = append_entry(values, var)) < 0)
-				break;
-			else
-				result = 0;
-
-			/* Add or append the new config option */
-			if (!git__strcmp(var->entry->name, "include.path")) {
-				struct reader *r;
-				git_buf path = GIT_BUF_INIT;
-				char *dir;
-				uint32_t index;
-
-				r = git_array_alloc(cfg_file->readers);
-				/* The reader may have been reallocated */
-				reader = git_array_get(cfg_file->readers, reader_idx);
-				memset(r, 0, sizeof(struct reader));
-				if ((result = git_path_dirname_r(&path, reader->file_path)) < 0)
-					break;
-
-				/* We need to know our index in the array, as the next config_parse call may realloc */
-				index = git_array_size(cfg_file->readers) - 1;
-				dir = git_buf_detach(&path);
-				result = included_path(&path, dir, var->entry->value);
-				git__free(dir);
-
-				if (result < 0)
-					break;
-
-				r->file_path = git_buf_detach(&path);
-				git_buf_init(&r->buffer, 0);
-				result = git_futils_readbuffer_updated(&r->buffer, r->file_path, &r->file_mtime,
-									    &r->file_size, NULL);
-
-				if (result == 0) {
-					result = config_parse(values, cfg_file, r, level, depth+1);
-					r = git_array_get(cfg_file->readers, index);
-					reader = git_array_get(cfg_file->readers, reader_idx);
-				}
-				else if (result == GIT_ENOTFOUND) {
-					giterr_clear();
-					result = 0;
-				}
-
-				git_buf_free(&r->buffer);
-
-				if (result < 0)
-					break;
-			}
-
-			break;
-		}
-	}
-
-	git__free(current_section);
-	return result;
-}
-
-static int write_section(git_filebuf *file, const char *key)
-{
-	int result;
-	const char *dot;
-	git_buf buf = GIT_BUF_INIT;
-
-	/* All of this just for [section "subsection"] */
-	dot = strchr(key, '.');
-	git_buf_putc(&buf, '[');
-	if (dot == NULL) {
-		git_buf_puts(&buf, key);
-	} else {
-		char *escaped;
-		git_buf_put(&buf, key, dot - key);
-		escaped = escape_value(dot + 1);
-		GITERR_CHECK_ALLOC(escaped);
-		git_buf_printf(&buf, " \"%s\"", escaped);
-		git__free(escaped);
-	}
-	git_buf_puts(&buf, "]\n");
-
-	if (git_buf_oom(&buf))
-		return -1;
-
-	result = git_filebuf_write(file, git_buf_cstr(&buf), buf.size);
-	git_buf_free(&buf);
-
-	return result;
-}
-
-static const char *quotes_for_value(const char *value)
-{
-	const char *ptr;
-
-	if (value[0] == ' ' || value[0] == '\0')
-		return "\"";
-
-	for (ptr = value; *ptr; ++ptr) {
-		if (*ptr == ';' || *ptr == '#')
-			return "\"";
-	}
-
-	if (ptr[-1] == ' ')
-		return "\"";
-
-	return "";
-}
-
-/*
- * This is pretty much the parsing, except we write out anything we don't have
- */
-static int config_write(diskfile_backend *cfg, const char *key, const regex_t *preg, const char* value)
-{
-	int result, c;
-	int section_matches = 0, last_section_matched = 0, preg_replaced = 0, write_trailer = 0;
-	const char *pre_end = NULL, *post_start = NULL, *data_start, *write_start;
-	char *current_section = NULL, *section, *name, *ldot;
-	git_filebuf file = GIT_FILEBUF_INIT;
-	struct reader *reader = git_array_get(cfg->readers, 0);
-
-	/* We need to read in our own config file */
-	result = git_futils_readbuffer(&reader->buffer, cfg->file_path);
-
-	/* Initialise the reading position */
-	if (result == GIT_ENOTFOUND) {
-		reader->read_ptr = NULL;
-		reader->eof = 1;
-		data_start = NULL;
-		git_buf_clear(&reader->buffer);
-	} else if (result == 0) {
-		reader->read_ptr = reader->buffer.ptr;
-		reader->eof = 0;
-		data_start = reader->read_ptr;
-	} else {
-		return -1; /* OS error when reading the file */
-	}
-
-	write_start = data_start;
-
-	/* Lock the file */
-	if ((result = git_filebuf_open(
-		&file, cfg->file_path, 0, GIT_CONFIG_FILE_MODE)) < 0) {
-			git_buf_free(&reader->buffer);
-			return result;
-	}
-
-	skip_bom(reader);
-	ldot = strrchr(key, '.');
-	name = ldot + 1;
-	section = git__strndup(key, ldot - key);
-
-	while (!reader->eof) {
-		c = reader_peek(reader, SKIP_WHITESPACE);
-
-		if (c == '\n') { /* We've arrived at the end of the file */
-			break;
-
-		} else if (c == '[') { /* section header, new section begins */
-			/*
-			 * We set both positions to the current one in case we
-			 * need to add a variable to the end of a section. In that
-			 * case, we want both variables to point just before the
-			 * new section. If we actually want to replace it, the
-			 * default case will take care of updating them.
-			 */
-			pre_end = post_start = reader->read_ptr;
-
-			git__free(current_section);
-			current_section = NULL;
-			if (parse_section_header(reader, &current_section) < 0)
-				goto rewrite_fail;
-
-			/* Keep track of when it stops matching */
-			last_section_matched = section_matches;
-			section_matches = !strcmp(current_section, section);
-		}
-
-		else if (c == ';' || c == '#') {
-			reader_consume_line(reader);
-		}
-
-		else {
-			/*
-			 * If the section doesn't match, but the last section did,
-			 * it means we need to add a variable (so skip the line
-			 * otherwise). If both the section and name match, we need
-			 * to overwrite the variable (so skip the line
-			 * otherwise). pre_end needs to be updated each time so we
-			 * don't loose that information, but we only need to
-			 * update post_start if we're going to use it in this
-			 * iteration.
-			 * If the section doesn't match and we are trying to delete an entry
-			 * (value == NULL), we must continue searching; there may be another
-			 * matching section later.
-			 */
-			if (!section_matches) {
-				if (!last_section_matched || value == NULL) {
-					reader_consume_line(reader);
-					continue;
-				}
-			} else {
-				int has_matched = 0;
-				char *var_name, *var_value;
-
-				pre_end = reader->read_ptr;
-				if (parse_variable(reader, &var_name, &var_value) < 0)
-					goto rewrite_fail;
-
-				/* First try to match the name of the variable */
-				if (strcasecmp(name, var_name) == 0)
-					has_matched = 1;
-
-				/* If the name matches, and we have a regex to match the
-				 * value, try to match it */
-				if (has_matched && preg != NULL)
-					has_matched = (regexec(preg, var_value, 0, NULL, 0) == 0);
-
-				git__free(var_name);
-				git__free(var_value);
-
-				/* if there is no match, keep going */
-				if (!has_matched)
-					continue;
-
-				post_start = reader->read_ptr;
-			}
-
-			/* We've found the variable we wanted to change, so
-			 * write anything up to it */
-			git_filebuf_write(&file, write_start, pre_end - write_start);
-			preg_replaced = 1;
-
-			/* Then replace the variable. If the value is NULL, it
-			 * means we want to delete it, so don't write anything. */
-			if (value != NULL) {
-				const char *q = quotes_for_value(value);
-				git_filebuf_printf(&file, "\t%s = %s%s%s\n", name, q, value, q);
-			}
-
-			/*
-			 * If we have a multivar, we should keep looking for entries,
-			 * but only if we're in the right section. Otherwise we'll end up
-			 * looping on the edge of a matching and a non-matching section.
-			 */
-			if (section_matches && preg != NULL) {
-				write_start = post_start;
-				continue;
-			}
-
-			write_trailer = 1;
-			break; /* break from the loop */
-		}
-	}
-
-	/*
-	 * Being here can mean that
-	 *
-	 * 1) our section is the last one in the file and we're
-	 * adding a variable
-	 *
-	 * 2) we didn't find a section for us so we need to create it
-	 * ourselves.
-	 *
-	 * 3) we're setting a multivar with a regex, which means we
-	 * continue to search for matching values
-	 *
-	 * In the last case, if we've already replaced a value, we
-	 * want to write the rest of the file. Otherwise we need to write
-	 * out the whole file and then the new variable.
-	 */
-	if (write_trailer) {
-		/* Write out rest of the file */
-		git_filebuf_write(&file, post_start, reader->buffer.size - (post_start - data_start));
-	} else {
-		if (preg_replaced) {
-			git_filebuf_printf(&file, "\n%s", write_start);
-		} else {
-			const char *q;
-
-			git_filebuf_write(&file, reader->buffer.ptr, reader->buffer.size);
-
-			if (reader->buffer.size > 0 && *(reader->buffer.ptr + reader->buffer.size - 1) != '\n')
-				git_filebuf_write(&file, "\n", 1);
-
-			/* And now if we just need to add a variable */
-			if (!section_matches && write_section(&file, section) < 0)
-				goto rewrite_fail;
-
-			/* Sanity check: if we are here, and value is NULL, that means that somebody
-			 * touched the config file after our initial read. We should probably assert()
-			 * this, but instead we'll handle it gracefully with an error. */
-			if (value == NULL) {
-				giterr_set(GITERR_CONFIG,
-					"race condition when writing a config file (a cvar has been removed)");
-				goto rewrite_fail;
-			}
-
-			/* If we are here, there is at least a section line */
-			q = quotes_for_value(value);
-			git_filebuf_printf(&file, "\t%s = %s%s%s\n", name, q, value, q);
-		}
-	}
-
-	git__free(section);
-	git__free(current_section);
-
-	/* refresh stats - if this errors, then commit will error too */
-	(void)git_filebuf_stats(&reader->file_mtime, &reader->file_size, &file);
-
-	result = git_filebuf_commit(&file);
-	git_buf_free(&reader->buffer);
-
-	return result;
-
-rewrite_fail:
-	git__free(section);
-	git__free(current_section);
-
-	git_filebuf_cleanup(&file);
-	git_buf_free(&reader->buffer);
-	return -1;
-}
-
 static const char *escapes = "ntb\"\\";
 static const char *escaped = "\n\t\b\"\\";
 
@@ -1813,3 +1419,435 @@ on_error:
 	git__free(line);
 	return -1;
 }
+
+static int config_parse(
+	struct reader *reader,
+	int (*on_section)(struct reader **reader, const char *current_section, const char *line, size_t line_len, void *data),
+	int (*on_variable)(struct reader **reader, const char *current_section, char *var_name, char *var_value, const char *line, size_t line_len, void *data),
+	int (*on_comment)(struct reader **reader, const char *line, size_t line_len, void *data),
+	int (*on_eof)(struct reader **reader, void *data),
+	void *data)
+{
+	char *current_section = NULL, *var_name, *var_value, *line_start;
+	char c;
+	size_t line_len;
+	int result = 0;
+
+	skip_bom(reader);
+
+	while (result == 0 && !reader->eof) {
+		line_start = reader->read_ptr;
+
+		c = reader_peek(reader, SKIP_WHITESPACE);
+
+		switch (c) {
+		case '\0': /* EOF when peeking, set EOF in the reader to exit the loop */
+			reader->eof = 1;
+			break;
+
+		case '[': /* section header, new section begins */
+			git__free(current_section);
+			current_section = NULL;
+
+			if ((result = parse_section_header(reader, &current_section)) == 0 && on_section) {
+				line_len = reader->read_ptr - line_start;
+				result = on_section(&reader, current_section, line_start, line_len, data);
+			}
+			break;
+
+		case '\n': /* comment or whitespace-only */
+		case ';':
+		case '#':
+			reader_consume_line(reader);
+
+			if (on_comment) {
+				line_len = reader->read_ptr - line_start;
+				result = on_comment(&reader, line_start, line_len, data);
+			}
+			break;
+
+		default: /* assume variable declaration */
+			if ((result = parse_variable(reader, &var_name, &var_value)) == 0 && on_variable) {
+				line_len = reader->read_ptr - line_start;
+				result = on_variable(&reader, current_section, var_name, var_value, line_start, line_len, data);
+			}
+			break;
+		}
+	}
+
+	if (on_eof)
+		result = on_eof(&reader, data);
+
+	git__free(current_section);
+	return result;
+}
+
+struct parse_data {
+	git_strmap *values;
+	diskfile_backend *cfg_file;
+	uint32_t reader_idx;
+	git_config_level_t level;
+	int depth;
+};
+
+static int read_on_variable(
+	struct reader **reader,
+	const char *current_section,
+	char *var_name,
+	char *var_value,
+	const char *line,
+	size_t line_len,
+	void *data)
+{
+	struct parse_data *parse_data = (struct parse_data *)data;
+	git_buf buf = GIT_BUF_INIT;
+	cvar_t *var;
+	int result = 0;
+
+	GIT_UNUSED(line);
+	GIT_UNUSED(line_len);
+
+	git__strtolower(var_name);
+	git_buf_printf(&buf, "%s.%s", current_section, var_name);
+	git__free(var_name);
+
+	if (git_buf_oom(&buf)) {
+		git__free(var_value);
+		return -1;
+	}
+
+	var = git__calloc(1, sizeof(cvar_t));
+	GITERR_CHECK_ALLOC(var);
+	var->entry = git__calloc(1, sizeof(git_config_entry));
+	GITERR_CHECK_ALLOC(var->entry);
+
+	var->entry->name = git_buf_detach(&buf);
+	var->entry->value = var_value;
+	var->entry->level = parse_data->level;
+	var->included = !!parse_data->depth;
+
+	if ((result = append_entry(parse_data->values, var)) < 0)
+		return result;
+
+	result = 0;
+
+	/* Add or append the new config option */
+	if (!git__strcmp(var->entry->name, "include.path")) {
+		struct reader *r;
+		git_buf path = GIT_BUF_INIT;
+		char *dir;
+		uint32_t index;
+
+		r = git_array_alloc(parse_data->cfg_file->readers);
+		/* The reader may have been reallocated */
+		*reader = git_array_get(parse_data->cfg_file->readers, parse_data->reader_idx);
+		memset(r, 0, sizeof(struct reader));
+
+		if ((result = git_path_dirname_r(&path, (*reader)->file_path)) < 0)
+			return result;
+
+		/* We need to know our index in the array, as the next config_parse call may realloc */
+		index = git_array_size(parse_data->cfg_file->readers) - 1;
+		dir = git_buf_detach(&path);
+		result = included_path(&path, dir, var->entry->value);
+		git__free(dir);
+
+		if (result < 0)
+			return result;
+
+		r->file_path = git_buf_detach(&path);
+		git_buf_init(&r->buffer, 0);
+
+		result = git_futils_readbuffer_updated(
+			&r->buffer, r->file_path, &r->file_mtime, &r->file_size, NULL);
+
+		if (result == 0) {
+			result = config_read(parse_data->values, parse_data->cfg_file, r, parse_data->level, parse_data->depth+1);
+			r = git_array_get(parse_data->cfg_file->readers, index);
+			*reader = git_array_get(parse_data->cfg_file->readers, parse_data->reader_idx);
+		} else if (result == GIT_ENOTFOUND) {
+			giterr_clear();
+			result = 0;
+		}
+
+		git_buf_free(&r->buffer);
+	}
+
+	return result;
+}
+
+static int config_read(git_strmap *values, diskfile_backend *cfg_file, struct reader *reader, git_config_level_t level, int depth)
+{
+	struct parse_data parse_data;
+
+	if (depth >= MAX_INCLUDE_DEPTH) {
+		giterr_set(GITERR_CONFIG, "Maximum config include depth reached");
+		return -1;
+	}
+
+	/* Initialize the reading position */
+	reader->read_ptr = reader->buffer.ptr;
+	reader->eof = 0;
+
+	/* If the file is empty, there's nothing for us to do */
+	if (*reader->read_ptr == '\0')
+		return 0;
+
+	parse_data.values = values;
+	parse_data.cfg_file = cfg_file;
+	parse_data.reader_idx = git_array_size(cfg_file->readers) - 1;
+	parse_data.level = level;
+	parse_data.depth = depth;
+
+	return config_parse(reader, NULL, read_on_variable, NULL, NULL, &parse_data);
+}
+
+static int write_section(git_filebuf *file, const char *key)
+{
+	int result;
+	const char *dot;
+	git_buf buf = GIT_BUF_INIT;
+
+	/* All of this just for [section "subsection"] */
+	dot = strchr(key, '.');
+	git_buf_putc(&buf, '[');
+	if (dot == NULL) {
+		git_buf_puts(&buf, key);
+	} else {
+		char *escaped;
+		git_buf_put(&buf, key, dot - key);
+		escaped = escape_value(dot + 1);
+		GITERR_CHECK_ALLOC(escaped);
+		git_buf_printf(&buf, " \"%s\"", escaped);
+		git__free(escaped);
+	}
+	git_buf_puts(&buf, "]\n");
+
+	if (git_buf_oom(&buf))
+		return -1;
+
+	result = git_filebuf_write(file, git_buf_cstr(&buf), buf.size);
+	git_buf_free(&buf);
+
+	return result;
+}
+
+static const char *quotes_for_value(const char *value)
+{
+	const char *ptr;
+
+	if (value[0] == ' ' || value[0] == '\0')
+		return "\"";
+
+	for (ptr = value; *ptr; ++ptr) {
+		if (*ptr == ';' || *ptr == '#')
+			return "\"";
+	}
+
+	if (ptr[-1] == ' ')
+		return "\"";
+
+	return "";
+}
+
+struct write_data {
+	git_filebuf *file;
+	unsigned int in_section : 1,
+		preg_replaced : 1;
+	const char *section;
+	const char *name;
+	const regex_t *preg;
+	const char *value;
+};
+
+static int write_line(struct write_data *write_data, const char *line, size_t line_len)
+{
+	int result = git_filebuf_write(write_data->file, line, line_len);
+
+	if (!result && line_len && line[line_len-1] != '\n')
+		result = git_filebuf_printf(write_data->file, "\n");
+
+	return result;
+}
+
+static int write_value(struct write_data *write_data)
+{
+	const char *q;
+	int result;
+
+	q = quotes_for_value(write_data->value);
+	result = git_filebuf_printf(write_data->file,
+		"\t%s = %s%s%s\n", write_data->name, q, write_data->value, q);
+
+	/* If we are updating a single name/value, we're done.  Setting `value`
+	 * to `NULL` will prevent us from trying to write it again later (in
+	 * `write_on_section`) if we see the same section repeated.
+	 */
+	if (!write_data->preg)
+		write_data->value = NULL;
+
+	return result;
+}
+
+static int write_on_section(
+	struct reader **reader,
+	const char *current_section,
+	const char *line,
+	size_t line_len,
+	void *data)
+{
+	struct write_data *write_data = (struct write_data *)data;
+	int result = 0;
+
+	GIT_UNUSED(reader);
+
+	/* If we were previously in the correct section (but aren't anymore)
+	 * and haven't written our value (for a simple name/value set, not
+	 * a multivar), then append it to the end of the section before writing
+	 * the new one.
+	 */
+	if (write_data->in_section && !write_data->preg && write_data->value)
+		result = write_value(write_data);
+
+	write_data->in_section = strcmp(current_section, write_data->section) == 0;
+
+	if (!result)
+		result = write_line(write_data, line, line_len);
+
+	return result;
+}
+
+static int write_on_variable(
+	struct reader **reader,
+	const char *current_section,
+	char *var_name,
+	char *var_value,
+	const char *line,
+	size_t line_len,
+	void *data)
+{
+	struct write_data *write_data = (struct write_data *)data;
+	bool has_matched = false;
+
+	GIT_UNUSED(reader);
+	GIT_UNUSED(current_section);
+
+	/* See if we are to update this name/value pair; first examine name */
+	if (write_data->in_section &&
+		strcasecmp(write_data->name, var_name) == 0)
+		has_matched = true;
+
+	/* If we have a regex to match the value, see if it matches */
+	if (has_matched && write_data->preg != NULL)
+		has_matched = (regexec(write_data->preg, var_value, 0, NULL, 0) == 0);
+
+	git__free(var_name);
+	git__free(var_value);
+
+	/* If this isn't the name/value we're looking for, simply dump the
+	 * existing data back out and continue on.
+	 */
+	if (!has_matched)
+		return write_line(write_data, line, line_len);
+
+	write_data->preg_replaced = 1;
+
+	/* If value is NULL, we are deleting this value; write nothing. */
+	if (!write_data->value)
+		return 0;
+
+	return write_value(write_data);
+}
+
+static int write_on_comment(struct reader **reader, const char *line, size_t line_len, void *data)
+{
+	struct write_data *write_data;
+
+	GIT_UNUSED(reader);
+
+	write_data = (struct write_data *)data;
+	return write_line(write_data, line, line_len);
+}
+
+static int write_on_eof(struct reader **reader, void *data)
+{
+	struct write_data *write_data = (struct write_data *)data;
+	int result = 0;
+
+	GIT_UNUSED(reader);
+
+	/* If we are at the EOF and have not written our value (again, for a
+	 * simple name/value set, not a multivar) then we have never seen the
+	 * section in question and should create a new section and write the
+	 * value.
+	 */
+	if ((!write_data->preg || !write_data->preg_replaced) && write_data->value) {
+		if ((result = write_section(write_data->file, write_data->section)) == 0)
+			result = write_value(write_data);
+	}
+
+	return result;
+}
+
+/*
+ * This is pretty much the parsing, except we write out anything we don't have
+ */
+static int config_write(diskfile_backend *cfg, const char *key, const regex_t *preg, const char* value)
+{
+	int result;
+	char *section, *name, *ldot;
+	git_filebuf file = GIT_FILEBUF_INIT;
+	struct reader *reader = git_array_get(cfg->readers, 0);
+	struct write_data write_data;
+
+	/* Lock the file */
+	if ((result = git_filebuf_open(
+		&file, cfg->file_path, 0, GIT_CONFIG_FILE_MODE)) < 0) {
+			git_buf_free(&reader->buffer);
+			return result;
+	}
+
+	/* We need to read in our own config file */
+	result = git_futils_readbuffer(&reader->buffer, cfg->file_path);
+
+	/* Initialise the reading position */
+	if (result == GIT_ENOTFOUND) {
+		reader->read_ptr = NULL;
+		reader->eof = 1;
+		git_buf_clear(&reader->buffer);
+	} else if (result == 0) {
+		reader->read_ptr = reader->buffer.ptr;
+		reader->eof = 0;
+	} else {
+		git_filebuf_cleanup(&file);
+		return -1; /* OS error when reading the file */
+	}
+
+	ldot = strrchr(key, '.');
+	name = ldot + 1;
+	section = git__strndup(key, ldot - key);
+
+	write_data.file = &file;
+	write_data.section = section;
+	write_data.in_section = 0;
+	write_data.preg_replaced = 0;
+	write_data.name = name;
+	write_data.preg = preg;
+	write_data.value = value;
+
+	if ((result = config_parse(reader, write_on_section, write_on_variable, write_on_comment, write_on_eof, &write_data)) < 0) {
+		git_filebuf_cleanup(&file);
+		goto done;
+	}
+
+	/* refresh stats - if this errors, then commit will error too */
+	(void)git_filebuf_stats(&reader->file_mtime, &reader->file_size, &file);
+
+	result = git_filebuf_commit(&file);
+	git_buf_free(&reader->buffer);
+
+done:
+	git_buf_free(&reader->buffer);
+	return result;
+}
+
diff --git a/tests/config/write.c b/tests/config/write.c
index 1832466..2e7b818 100644
--- a/tests/config/write.c
+++ b/tests/config/write.c
@@ -145,6 +145,138 @@ void test_config_write__delete_value_with_duplicate_header(void)
 	git_config_free(cfg);
 }
 
+/*
+ * This test exposes a bug where duplicate section headers could cause
+ * config_write to add a new entry when one already exists.
+ */
+void test_config_write__add_value_with_duplicate_header(void)
+{
+	const char *file_name  = "config-duplicate-insert";
+	const char *entry_name = "foo.c";
+	const char *old_val    = "old";
+	const char *new_val    = "new";
+	const char *str;
+	git_config *cfg, *snapshot;
+
+	/* c = old should be replaced by c = new.
+	 * The bug causes c = new to be inserted under the first 'foo' header.
+	 */
+	const char *file_content =
+		"[foo]\n"   \
+		"  a = b\n" \
+		"[other]\n" \
+		"  a = b\n" \
+		"[foo]\n"   \
+		"  c = old\n";
+
+	/* Write the test config */
+	cl_git_mkfile(file_name, file_content);
+	cl_git_pass(git_config_open_ondisk(&cfg, file_name));
+
+	/* make sure the expected entry (foo.c) exists */
+	cl_git_pass(git_config_snapshot(&snapshot, cfg));
+	cl_git_pass(git_config_get_string(&str, snapshot, entry_name));
+	cl_assert_equal_s(old_val, str);
+	git_config_free(snapshot);
+
+	/* Try setting foo.c to something else */
+	cl_git_pass(git_config_set_string(cfg, entry_name, new_val));
+	git_config_free(cfg);
+
+	/* Reopen the file and make sure the new value was set */
+	cl_git_pass(git_config_open_ondisk(&cfg, file_name));
+	cl_git_pass(git_config_snapshot(&snapshot, cfg));
+	cl_git_pass(git_config_get_string(&str, snapshot, entry_name));
+	cl_assert_equal_s(new_val, str);
+
+	/* Cleanup */
+	git_config_free(snapshot);
+	git_config_free(cfg);
+}
+
+void test_config_write__overwrite_value_with_duplicate_header(void)
+{
+	const char *file_name  = "config-duplicate-header";
+	const char *entry_name = "remote.origin.url";
+	git_config *cfg;
+	git_config_entry *entry;
+
+	/* This config can occur after removing and re-adding the origin remote */
+	const char *file_content =
+		"[remote \"origin\"]\n"		\
+		"[branch \"master\"]\n"		\
+		"	remote = \"origin\"\n"	\
+		"[remote \"origin\"]\n"		\
+		"	url = \"foo\"\n";
+
+	/* Write the test config and make sure the expected entry exists */
+	cl_git_mkfile(file_name, file_content);
+	cl_git_pass(git_config_open_ondisk(&cfg, file_name));
+	cl_git_pass(git_config_get_entry(&entry, cfg, entry_name));
+
+	/* Update that entry */
+	cl_git_pass(git_config_set_string(cfg, entry_name, "newurl"));
+
+	/* Reopen the file and make sure the entry was updated */
+	git_config_entry_free(entry);
+	git_config_free(cfg);
+	cl_git_pass(git_config_open_ondisk(&cfg, file_name));
+	cl_git_pass(git_config_get_entry(&entry, cfg, entry_name));
+
+	cl_assert_equal_s("newurl", entry->value);
+
+	/* Cleanup */
+	git_config_entry_free(entry);
+	git_config_free(cfg);
+}
+
+static int multivar_cb(const git_config_entry *entry, void *data)
+{
+	int *n = (int *)data;
+
+	cl_assert_equal_s(entry->value, "newurl");
+
+	(*n)++;
+
+	return 0;
+}
+
+void test_config_write__overwrite_multivar_within_duplicate_header(void)
+{
+	const char *file_name  = "config-duplicate-header";
+	const char *entry_name = "remote.origin.url";
+	git_config *cfg;
+	git_config_entry *entry;
+	int n = 0;
+
+	/* This config can occur after removing and re-adding the origin remote */
+	const char *file_content =
+		"[remote \"origin\"]\n"		\
+		"	url = \"bar\"\n"		\
+		"[branch \"master\"]\n"		\
+		"	remote = \"origin\"\n"	\
+		"[remote \"origin\"]\n"		\
+		"	url = \"foo\"\n";
+
+	/* Write the test config and make sure the expected entry exists */
+	cl_git_mkfile(file_name, file_content);
+	cl_git_pass(git_config_open_ondisk(&cfg, file_name));
+	cl_git_pass(git_config_get_entry(&entry, cfg, entry_name));
+
+	/* Update that entry */
+	cl_git_pass(git_config_set_multivar(cfg, entry_name, ".*", "newurl"));
+	git_config_entry_free(entry);
+	git_config_free(cfg);
+
+	/* Reopen the file and make sure the entry was updated */
+	cl_git_pass(git_config_open_ondisk(&cfg, file_name));
+	cl_git_pass(git_config_get_multivar_foreach(cfg, entry_name, NULL, multivar_cb, &n));
+	cl_assert_equal_i(2, n);
+
+	/* Cleanup */
+	git_config_free(cfg);
+}
+
 void test_config_write__write_subsection(void)
 {
 	git_config *cfg;
@@ -395,6 +527,75 @@ void test_config_write__outside_change(void)
 	git_config_free(cfg);
 }
 
+#define SECTION_FOO \
+	"\n"                     \
+	"    \n"                 \
+	" [section \"foo\"]  \n" \
+	" # here's a comment\n"  \
+	"\tname = \"value\"\n"   \
+	"  name2 = \"value2\"\n" \
+	";  another comment!\n"
+
+#define SECTION_BAR \
+	"[section \"bar\"]\t\n"  \
+	"\t  \n"                 \
+	" barname=\"value\"\n"
+
+
+void test_config_write__preserves_whitespace_and_comments(void)
+{
+	const char *file_name  = "config-duplicate-header";
+	const char *n;
+	git_config *cfg;
+	git_buf newfile = GIT_BUF_INIT;
+
+	/* This config can occur after removing and re-adding the origin remote */
+	const char *file_content = SECTION_FOO SECTION_BAR;
+
+	/* Write the test config and make sure the expected entry exists */
+	cl_git_mkfile(file_name, file_content);
+	cl_git_pass(git_config_open_ondisk(&cfg, file_name));
+	cl_git_pass(git_config_set_string(cfg, "section.foo.other", "otherval"));
+	cl_git_pass(git_config_set_string(cfg, "newsection.newname", "new_value"));
+
+	/* Ensure that we didn't needlessly mangle the config file */
+	cl_git_pass(git_futils_readbuffer(&newfile, file_name));
+	n = newfile.ptr;
+
+	cl_assert_equal_strn(SECTION_FOO, n, strlen(SECTION_FOO));
+	n += strlen(SECTION_FOO);
+
+	cl_assert_equal_strn("\tother = otherval\n", n, strlen("\tother = otherval\n"));
+	n += strlen("\tother = otherval\n");
+
+	cl_assert_equal_strn(SECTION_BAR, n, strlen(SECTION_BAR));
+	n += strlen(SECTION_BAR);
+
+	cl_assert_equal_s("[newsection]\n\tnewname = new_value\n", n);
+
+	git_buf_free(&newfile);
+	git_config_free(cfg);
+}
+
+void test_config_write__preserves_entry_with_name_only(void)
+{
+	const char *file_name  = "config-empty-value";
+	git_config *cfg;
+	git_buf newfile = GIT_BUF_INIT;
+
+	/* Write the test config and make sure the expected entry exists */
+	cl_git_mkfile(file_name, "[section \"foo\"]\n\tname\n");
+	cl_git_pass(git_config_open_ondisk(&cfg, file_name));
+	cl_git_pass(git_config_set_string(cfg, "newsection.newname", "new_value"));
+	cl_git_pass(git_config_set_string(cfg, "section.foo.other", "otherval"));
+
+	cl_git_pass(git_futils_readbuffer(&newfile, file_name));
+	cl_assert_equal_s("[section \"foo\"]\n\tname\n\tother = otherval\n[newsection]\n\tnewname = new_value\n", newfile.ptr);
+
+	git_buf_free(&newfile);
+	git_config_free(cfg);
+}
+
 void test_config_write__to_empty_file(void)
 {
 	git_config *cfg;
@@ -428,3 +629,4 @@ void test_config_write__to_file_with_only_comment(void)
 
 	git_buf_free(&result);
 }
+