Windows: Implement start_command().

On Windows, we have spawnv() variants to run a child process instead of
fork()/exec(). In order to attach pipe ends to stdin, stdout, and stderr,
we have to use this idiom:

    save1 = dup(1);
    dup2(pipe[1], 1);
    spawnv();
    dup2(save1, 1);
    close(pipe[1]);

assuming that the descriptors created by pipe() are not inheritable.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>

compat/mingw.c

@@ -315,6 +315,68 @@ void mingw_execvp(const char *cmd, char *const *argv)
 	free_path_split(path);
 }
 
+char **copy_environ()
+{
+	char **env;
+	int i = 0;
+	while (environ[i])
+		i++;
+	env = xmalloc((i+1)*sizeof(*env));
+	for (i = 0; environ[i]; i++)
+		env[i] = xstrdup(environ[i]);
+	env[i] = NULL;
+	return env;
+}
+
+void free_environ(char **env)
+{
+	int i;
+	for (i = 0; env[i]; i++)
+		free(env[i]);
+	free(env);
+}
+
+static int lookup_env(char **env, const char *name, size_t nmln)
+{
+	int i;
+
+	for (i = 0; env[i]; i++) {
+		if (0 == strncmp(env[i], name, nmln)
+		    && '=' == env[i][nmln])
+			/* matches */
+			return i;
+	}
+	return -1;
+}
+
+/*
+ * If name contains '=', then sets the variable, otherwise it unsets it
+ */
+char **env_setenv(char **env, const char *name)
+{
+	char *eq = strchrnul(name, '=');
+	int i = lookup_env(env, name, eq-name);
+
+	if (i < 0) {
+		if (*eq) {
+			for (i = 0; env[i]; i++)
+				;
+			env = xrealloc(env, (i+2)*sizeof(*env));
+			env[i] = xstrdup(name);
+			env[i+1] = NULL;
+		}
+	}
+	else {
+		free(env[i]);
+		if (*eq)
+			env[i] = xstrdup(name);
+		else
+			for (; env[i]; i++)
+				env[i] = env[i+1];
+	}
+	return env;
+}
+
 #undef rename
 int mingw_rename(const char *pold, const char *pnew)
 {

compat/mingw.h

@@ -165,3 +165,11 @@ sig_handler_t mingw_signal(int sig, sig_handler_t handler);
 #define is_dir_sep(c) ((c) == '/' || (c) == '\\')
 #define PATH_SEP ';'
 #define PRIuMAX "I64u"
+
+/*
+ * helpers
+ */
+
+char **copy_environ(void);
+void free_environ(char **env);
+char **env_setenv(char **env, const char *name);

run-command.c

@@ -65,21 +65,8 @@ int start_command(struct child_process *cmd)
 		cmd->err = fderr[0];
 	}
 
+#ifndef __MINGW32__
 	cmd->pid = fork();
-	if (cmd->pid < 0) {
-		if (need_in)
-			close_pair(fdin);
-		else if (cmd->in)
-			close(cmd->in);
-		if (need_out)
-			close_pair(fdout);
-		else if (cmd->out)
-			close(cmd->out);
-		if (need_err)
-			close_pair(fderr);
-		return -ERR_RUN_COMMAND_FORK;
-	}
-
 	if (!cmd->pid) {
 		if (cmd->no_stdin)
 			dup_devnull(0);
@@ -128,6 +115,88 @@ int start_command(struct child_process *cmd)
 		}
 		die("exec %s failed.", cmd->argv[0]);
 	}
+#else
+	int s0 = -1, s1 = -1, s2 = -1;	/* backups of stdin, stdout, stderr */
+	const char *sargv0 = cmd->argv[0];
+	char **env = environ;
+	struct strbuf git_cmd;
+
+	if (cmd->no_stdin) {
+		s0 = dup(0);
+		dup_devnull(0);
+	} else if (need_in) {
+		s0 = dup(0);
+		dup2(fdin[0], 0);
+	} else if (cmd->in) {
+		s0 = dup(0);
+		dup2(cmd->in, 0);
+	}
+
+	if (cmd->no_stderr) {
+		s2 = dup(2);
+		dup_devnull(2);
+	} else if (need_err) {
+		s2 = dup(2);
+		dup2(fderr[1], 2);
+	}
+
+	if (cmd->no_stdout) {
+		s1 = dup(1);
+		dup_devnull(1);
+	} else if (cmd->stdout_to_stderr) {
+		s1 = dup(1);
+		dup2(2, 1);
+	} else if (need_out) {
+		s1 = dup(1);
+		dup2(fdout[1], 1);
+	} else if (cmd->out > 1) {
+		s1 = dup(1);
+		dup2(cmd->out, 1);
+	}
+
+	if (cmd->dir)
+		die("chdir in start_command() not implemented");
+	if (cmd->env) {
+		env = copy_environ();
+		for (; *cmd->env; cmd->env++)
+			env = env_setenv(env, *cmd->env);
+	}
+
+	if (cmd->git_cmd) {
+		strbuf_init(&git_cmd, 0);
+		strbuf_addf(&git_cmd, "git-%s", cmd->argv[0]);
+		cmd->argv[0] = git_cmd.buf;
+	}
+
+	cmd->pid = spawnvpe(_P_NOWAIT, cmd->argv[0], cmd->argv, (const char **)env);
+
+	if (cmd->env)
+		free_environ(env);
+	if (cmd->git_cmd)
+		strbuf_release(&git_cmd);
+
+	cmd->argv[0] = sargv0;
+	if (s0 >= 0)
+		dup2(s0, 0), close(s0);
+	if (s1 >= 0)
+		dup2(s1, 1), close(s1);
+	if (s2 >= 0)
+		dup2(s2, 2), close(s2);
+#endif
+
+	if (cmd->pid < 0) {
+		if (need_in)
+			close_pair(fdin);
+		else if (cmd->in)
+			close(cmd->in);
+		if (need_out)
+			close_pair(fdout);
+		else if (cmd->out)
+			close(cmd->out);
+		if (need_err)
+			close_pair(fderr);
+		return -ERR_RUN_COMMAND_FORK;
+	}
 
 	if (need_in)
 		close(fdin[0]);