[PATCH 06/11] libxl: Expose functions for helping with subprocesses.

[Ian Jackson <ian.jackson@eu.citrix.com>]

 * Expose libxl_fork in libxl_utils.h
 * Expose libxl_pipe in libxl_utils.h
 * Make libxl_exec put SIGPIPE back (so that libxl callers may
    have SIGPIPE ignored)

xl would like to use libxl_fork (which is like fork(2) except that it
logs errors) and also a similar function libxl_pipe.  So put these in
libxl_utils.[ch] and use them in libxl.c as appropriate, to avoid
having to duplicate code between xl and libxl.

Also, make sure that subprocesses spawned by libxl have SIGPIPE set
back to SIG_DFL as they are entitled to expect.  This means that a
libxl caller which sets SIGPIPE to SIG_IGN is no longer buggy.  (This
is relevant for xl migration, because xl would like to be such a
caller.)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
---
 tools/libxl/libxl.c       |   12 +++---------
 tools/libxl/libxl_exec.c  |   18 +++++-------------
 tools/libxl/libxl_utils.c |   23 +++++++++++++++++++++++
 tools/libxl/libxl_utils.h |    4 ++++
 4 files changed, 35 insertions(+), 22 deletions(-)

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index 59859ab..711f0a0 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -1238,15 +1238,9 @@ int libxl_device_disk_add(struct libxl_ctx *ctx, uint32_t domid, libxl_device_di
                 char buf[1024], *dev;
                 dev = get_blktap2_device(ctx, disk->physpath, device_disk_string_of_phystype(disk->phystype));
                 if (dev == NULL) {
-                    if (pipe(p) < 0) {
-                        XL_LOG(ctx, XL_LOG_ERROR, "Failed to create a pipe");
-                        return -1;
-                    }
-                    rc = fork();
-                    if (rc < 0) {
-                        XL_LOG(ctx, XL_LOG_ERROR, "Failed to fork a new process");
-                        return -1;
-                    } else if (!rc) { /* child */
+                    rc= libxl_pipe(ctx, p);  if (rc==-1) return -1;
+                    rc= libxl_fork(ctx);  if (rc==-1) return -1;
+                    if (!rc) { /* child */
                         int null_r, null_w;
                         char *args[4];
                         args[0] = "tapdisk2";
diff --git a/tools/libxl/libxl_exec.c b/tools/libxl/libxl_exec.c
index e8cc74c..6d4a5c5 100644
--- a/tools/libxl/libxl_exec.c
+++ b/tools/libxl/libxl_exec.c
@@ -30,19 +30,6 @@
 #include "libxl.h"
 #include "libxl_internal.h"
 
-static pid_t libxl_fork(struct libxl_ctx *ctx)
-{
-    pid_t pid;
-
-    pid = fork();
-    if (pid == -1) {
-        XL_LOG_ERRNO(ctx, XL_LOG_ERROR, "fork failed");
-        return -1;
-    }
-
-    return pid;
-}
-
 static int call_waitpid(pid_t (*waitpid_cb)(pid_t, int *, int), pid_t pid, int *status, int options)
 {
     return (waitpid_cb) ? waitpid_cb(pid, status, options) : waitpid(pid, status, options);
@@ -61,6 +48,11 @@ void libxl_exec(int stdinfd, int stdoutfd, int stderrfd, char *arg0, char **args
         dup2(stderrfd, STDERR_FILENO);
     for (i = 4; i < 256; i++)
         close(i);
+
+    signal(SIGPIPE, SIG_DFL);
+    /* in case our caller set it to IGN.  subprocesses are entitled
+     * to assume they got DFL. */
+
     execv(arg0, args);
     _exit(-1);
 }
diff --git a/tools/libxl/libxl_utils.c b/tools/libxl/libxl_utils.c
index 3a1f095..c6989ce 100644
--- a/tools/libxl/libxl_utils.c
+++ b/tools/libxl/libxl_utils.c
@@ -280,3 +280,26 @@ int libxl_read_file_contents(struct libxl_ctx *ctx, const char *filename,
 
 READ_WRITE_EXACTLY(read, 1, /* */)
 READ_WRITE_EXACTLY(write, 0, const)
+
+
+pid_t libxl_fork(struct libxl_ctx *ctx)
+{
+    pid_t pid;
+
+    pid = fork();
+    if (pid == -1) {
+        XL_LOG_ERRNO(ctx, XL_LOG_ERROR, "fork failed");
+        return -1;
+    }
+
+    return pid;
+}
+
+int libxl_pipe(struct libxl_ctx *ctx, int pipes[2])
+{
+    if (pipe(pipes) < 0) {
+        XL_LOG(ctx, XL_LOG_ERROR, "Failed to create a pipe");
+        return -1;
+    }
+    return 0;
+}
diff --git a/tools/libxl/libxl_utils.h b/tools/libxl/libxl_utils.h
index 65e7e31..fe8b975 100644
--- a/tools/libxl/libxl_utils.h
+++ b/tools/libxl/libxl_utils.h
@@ -44,5 +44,9 @@ int libxl_write_exactly(struct libxl_ctx *ctx, int fd, const void *data,
    * logged using filename (which is only used for logging) and what
    * (which may be 0). */
     
+pid_t libxl_fork(struct libxl_ctx *ctx);
+int libxl_pipe(struct libxl_ctx *ctx, int pipes[2]);
+  /* Just like fork(2), pipe(2), but log errors. */
+
 #endif
 
-- 
1.5.6.5