Allow overriding which POSIX shell to use

Right now, the compiled-in default is used always, which can be
overridden at compile time via the `SHELL_PATH` variable.

However, in Git for Windows' context, there is a scenario where this is
not good enough: the BusyBox flavor of MinGit wants to use BusyBox'
`ash` instead.

To allow for that, let's introduce a new config variable: `core.shell`.
Its value is expected to be the absolute path to a valid, working POSIX
shell. This shell will be then used by `git.exe` whenever it needs to
execute something via shell, such as moderately complex aliases.

And it is not only Git that is expected to use that `ash`, it is also
OpenSSH (e.g. when running any configured `ProxyCommand`): programs
spawned from `git.exe` may need to run a shell and look at the
environment variable `SHELL` for that. Therefore, let's set that, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

Documentation/config/core.txt

@@ -628,3 +628,7 @@ core.abbrev::
 	If set to "no", no abbreviation is made and the object names
 	are shown in their full length.
 	The minimum length is 4.
+
+core.shell::
+	Set the absolute path to the executable to use as a POSIX shell;
+	This will also set/override the environment variable `SHELL`.

help.c

@@ -662,7 +662,7 @@ void get_version_info(struct strbuf *buf, int show_build_options)
 			strbuf_addstr(buf, "no commit associated with this build\n");
 		strbuf_addf(buf, "sizeof-long: %d\n", (int)sizeof(long));
 		strbuf_addf(buf, "sizeof-size_t: %d\n", (int)sizeof(size_t));
-		strbuf_addf(buf, "shell-path: %s\n", SHELL_PATH);
+		strbuf_addf(buf, "shell-path: %s\n", get_shell_path(SHELL_PATH));
 		/* NEEDSWORK: also save and output GIT-BUILD_OPTIONS? */
 	}
 }

run-command.c

@@ -9,6 +9,20 @@
 #include "quote.h"
 #include "config.h"
 
+const char *get_shell_path(const char *fallback)
+{
+	static const char *shell;
+	static int initialized;
+
+	if (!initialized) {
+		if (git_config_get_pathname("core.shell", &shell))
+			setenv("SHELL", shell, 1);
+		initialized = 1;
+	}
+
+	return shell ? shell : fallback;
+}
+
 void child_process_init(struct child_process *child)
 {
 	memset(child, 0, sizeof(*child));
@@ -271,9 +285,9 @@ static const char **prepare_shell_cmd(struct strvec *out, const char **argv)
 
 	if (strcspn(argv[0], "|&;<>()$`\\\"' \t\n*?[#~=%") != strlen(argv[0])) {
 #ifndef GIT_WINDOWS_NATIVE
-		strvec_push(out, SHELL_PATH);
+		strvec_push(out, get_shell_path(SHELL_PATH));
 #else
-		strvec_push(out, "sh");
+		strvec_push(out, get_shell_path("sh"));
 #endif
 		strvec_push(out, "-c");
 
@@ -411,7 +425,7 @@ static int prepare_cmd(struct strvec *out, const struct child_process *cmd)
 	 * Add SHELL_PATH so in the event exec fails with ENOEXEC we can
 	 * attempt to interpret the command with 'sh'.
 	 */
-	strvec_push(out, SHELL_PATH);
+	strvec_push(out, get_shell_path(SHELL_PATH));
 
 	if (cmd->git_cmd) {
 		prepare_git_cmd(out, cmd->argv);

run-command.h

@@ -483,4 +483,9 @@ int run_processes_parallel_tr2(int n, get_next_task_fn, start_failure_fn,
 			       task_finished_fn, void *pp_cb,
 			       const char *tr2_category, const char *tr2_label);
 
+/*
+ * Get the path of the POSIX shell to use in `start_command()`.
+ */
+const char *get_shell_path(const char *fallback);
+
 #endif