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.adoc

@@ -807,3 +807,7 @@ core.WSLCompat::
 	The default value is false. When set to true, Git will set the mode
 	bits of the file in the way of wsl, so that the executable flag of
 	files can be set or read correctly.
+
+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

@@ -793,7 +793,7 @@ 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? */
 
 #if defined WITH_RUST

run-command.c

@@ -1,4 +1,5 @@
 #define DISABLE_SIGN_COMPARE_WARNINGS
+#define USE_THE_REPOSITORY_VARIABLE
 
 #include "git-compat-util.h"
 #include "run-command.h"
@@ -18,6 +19,20 @@
 #include "packfile.h"
 #include "compat/nonblock.h"
 
+const char *get_shell_path(const char *fallback)
+{
+	static char *shell;
+	static int initialized;
+
+	if (!initialized) {
+		if (!repo_config_get_pathname(the_repository, "core.shell", &shell))
+			setenv("SHELL", shell, 1);
+		initialized = 1;
+	}
+
+	return shell ? shell : fallback;
+}
+
 void child_process_init(struct child_process *child)
 {
 	struct child_process blank = CHILD_PROCESS_INIT;
@@ -276,12 +291,19 @@ int sane_execvp(const char *file, char * const argv[])
 
 char *git_shell_path(void)
 {
+	const char *shell = get_shell_path(NULL);
+
+	if (shell)
+		return xstrdup(shell);
+
 #ifndef GIT_WINDOWS_NATIVE
 	return xstrdup(SHELL_PATH);
 #else
-	char *p = locate_in_PATH("sh");
-	convert_slashes(p);
-	return p;
+	{
+		char *p = locate_in_PATH("sh");
+		convert_slashes(p);
+		return p;
+	}
 #endif
 }
 
@@ -416,7 +438,7 @@ 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->args.v);

run-command.h

@@ -616,4 +616,9 @@ enum start_bg_result start_bg_command(struct child_process *cmd,
 
 int sane_execvp(const char *file, char *const argv[]);
 
+/*
+ * Get the path of the POSIX shell to use in `start_command()`.
+ */
+const char *get_shell_path(const char *fallback);
+
 #endif

t/t0061-run-command.sh

@@ -291,4 +291,11 @@ test_expect_success MINGW 'can spawn .bat with argv[0] containing spaces' '
 	test_grep "arg with spaces" out
 '
 
+SQ="'"
+test_expect_success 'core.shell' '
+	test_config_global core.shell "$GIT_EXEC_PATH/git$X" &&
+	test_must_fail git -c alias.eq="!a.b=c" eq 2>actual &&
+	grep "${SQ}a.b=c${SQ} is not a git command" actual
+'
+
 test_done