[PATCH 1/2] run-command: add pre-exec callback

[Jeff King <peff@peff.net>]

This is a function provided by the caller which is called
_after_ the process is forked, but before the spawned
program is executed. On platforms (like mingw) where
subprocesses are forked and executed in a single call, the
preexec callback is simply ignored.

This will be used in the following patch to do some setup
for 'less' that must happen in the forked child.

Signed-off-by: Jeff King <peff@peff.net>
---
 run-command.c |    2 ++
 run-command.h |    1 +
 2 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/run-command.c b/run-command.c
index 6e29fdf..73d0c31 100644
--- a/run-command.c
+++ b/run-command.c
@@ -110,6 +110,8 @@ int start_command(struct child_process *cmd)
 					unsetenv(*cmd->env);
 			}
 		}
+		if (cmd->preexec_cb)
+			cmd->preexec_cb();
 		if (cmd->git_cmd) {
 			execv_git_cmd(cmd->argv);
 		} else {
diff --git a/run-command.h b/run-command.h
index 5203a9e..4f2b7d7 100644
--- a/run-command.h
+++ b/run-command.h
@@ -42,6 +42,7 @@ struct child_process {
 	unsigned no_stderr:1;
 	unsigned git_cmd:1; /* if this is to be git sub-command */
 	unsigned stdout_to_stderr:1;
+	void (*preexec_cb)(void);
 };
 
 int start_command(struct child_process *);
-- 
1.6.0.rc0.1.g9291f.dirty