config: implement config writing

After each variable gets set, we store it in our list (not completely
in the right position, but the close enough). Then we write out the
new config file in the same way that git.git does it (keep the rest of
the file intact and insert or replace the variable in its line).

Overwriting variables and adding new ones is supported (even on new
sections), though deleting isn't yet.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

src/config_file.c

@@ -26,9 +26,11 @@
 #include "common.h"
 #include "config.h"
 #include "fileops.h"
+#include "filebuf.h"
 #include "git2/config.h"
 #include "git2/types.h"
 
+
 #include <ctype.h>
 
 typedef struct cvar_t {
@@ -98,6 +100,7 @@ typedef struct {
 
 static int config_parse(diskfile_backend *cfg_file);
 static int parse_variable(diskfile_backend *cfg, char **var_name, char **var_value);
+static int config_write(diskfile_backend *cfg, cvar_t *var);
 
 static void cvar_free(cvar_t *var)
 {
@@ -349,7 +352,7 @@ static int config_set(git_config_file *cfg, const char *name, const char *value)
 		free(existing->value);
 		existing->value = tmp;
 
-		return GIT_SUCCESS;
+		return config_write(b, existing);
 	}
 
 	/*
@@ -386,6 +389,7 @@ static int config_set(git_config_file *cfg, const char *name, const char *value)
 	}
 
 	CVAR_LIST_APPEND(&b->var_list, var);
+	error = config_write(b, var);
 
  out:
 	if (error < GIT_SUCCESS)
@@ -892,6 +896,173 @@ static int config_parse(diskfile_backend *cfg_file)
 	return error == GIT_SUCCESS ? GIT_SUCCESS : git__rethrow(error, "Failed to parse config");
 }
 
+static int write_section(git_filebuf *file, cvar_t *var)
+{
+	int error;
+
+	error = git_filebuf_printf(file, "[%s]\n", var->section);
+	if (error < GIT_SUCCESS)
+		return error;
+
+	error = git_filebuf_printf(file, "    %s = %s\n", var->name, var->value);
+	return error;
+}
+
+/*
+ * This is pretty much the parsing, except we write out anything we don't have
+ */
+static int config_write(diskfile_backend *cfg, cvar_t *var)
+{
+	int error = GIT_SUCCESS, c;
+	int section_matches = 0, last_section_matched = 0;
+	char *current_section = NULL;
+	char *var_name, *var_value, *data_start;
+	git_filebuf file;
+	const char *pre_end = NULL, *post_start = NULL;
+
+	/* We need to read in our own config file */
+	error = gitfo_read_file(&cfg->reader.buffer, cfg->file_path);
+	if (error < GIT_SUCCESS) {
+		return git__rethrow(error, "Failed to read existing config file %s", cfg->file_path);
+	}
+
+	/* Initialise the reading position */
+	cfg->reader.read_ptr = cfg->reader.buffer.data;
+	cfg->reader.eof = 0;
+	data_start = cfg->reader.read_ptr;
+
+	/* Lock the file */
+	error = git_filebuf_open(&file, cfg->file_path, 0);
+	if (error < GIT_SUCCESS)
+		return git__rethrow(error, "Failed to lock config file");
+
+	skip_bom(cfg);
+
+	while (error == GIT_SUCCESS && !cfg->reader.eof) {
+		c = cfg_peek(cfg, SKIP_WHITESPACE);
+
+		switch (c) {
+		case '\0': /* We've arrived at the end of the file */
+			break;
+
+		case '[': /* 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 = cfg->reader.read_ptr;
+			free(current_section);
+			error = parse_section_header(cfg, &current_section);
+			if (error < GIT_SUCCESS)
+				break;
+
+			/* Keep track of when it stops matching */
+			last_section_matched = section_matches;
+			section_matches = !strcmp(current_section, var->section);
+			break;
+
+		case ';':
+		case '#':
+			cfg_consume_line(cfg);
+			break;
+
+		default:
+			/*
+			 * 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 (!section_matches) {
+				if (!last_section_matched) {
+					cfg_consume_line(cfg);
+					break;
+				}
+			} else {
+				pre_end = cfg->reader.read_ptr;
+				error = parse_variable(cfg, &var_name, &var_value);
+				if (error < GIT_SUCCESS || strcasecmp(var->name, var_name))
+					break;
+				post_start = cfg->reader.read_ptr;
+			}
+
+			/*
+			 * We've found the variable we wanted to change, so
+			 * write anything up to it
+			 */
+			error = git_filebuf_write(&file, data_start, pre_end - data_start);
+			if (error < GIT_SUCCESS) {
+				git__rethrow(error, "Failed to write the first part of the file");
+				break;
+			}
+
+			/* Then replace the variable */
+			error = git_filebuf_printf(&file, "    %s = %s\n", var->name, var->value);
+			if (error < GIT_SUCCESS) {
+				git__rethrow(error, "Failed to overwrite the variable");
+				break;
+			}
+
+			/* And then the write out rest of the file */
+			error = git_filebuf_write(&file, post_start,
+			            cfg->reader.buffer.len - (post_start - data_start));
+
+			if (error < GIT_SUCCESS) {
+				git__rethrow(error, "Failed to write the rest of the file");
+					break;
+			}
+
+			goto cleanup;
+		}
+	}
+
+	/*
+	 * 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.
+	 *
+	 * Either way we need to write out the whole file.
+	 */
+
+	error = git_filebuf_write(&file, cfg->reader.buffer.data, cfg->reader.buffer.len);
+	if (error < GIT_SUCCESS) {
+		git__rethrow(error, "Failed to write original config content");
+		goto cleanup;
+	}
+
+	/* And now if we just need to add a variable */
+	if (section_matches) {
+		error = git_filebuf_printf(&file, "    %s = %s\n", var->name, var->value);
+		goto cleanup;
+	}
+
+	/* Or maybe we need to write out a whole section */
+	error = write_section(&file, var);
+	if (error < GIT_SUCCESS)
+		git__rethrow(error, "Failed to write new section");
+
+ cleanup:
+	free(current_section);
+
+	if (error < GIT_SUCCESS)
+		git_filebuf_cleanup(&file);
+	else
+		error = git_filebuf_commit(&file);
+
+	return error;
+}
+
 static int is_multiline_var(const char *str)
 {
 	char *end = strrchr(str, '\0') - 1;