[File] [PATCH 3/4] use vfork when spawning decompressor

[Denys Vlasenko]

Tools such as rpmdiff and rpmbuild call libmagic from processes
with large mapped virtual sizes (gigabytes). In this case,
vfork is much faster than fork (sometimes x100 faster).

Signed-off-by: Denys Vlasenko <dvlasenk at redhat.com>
---
 src/compress.c | 35 ++++++++++++++++++++++++++---------
 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/src/compress.c b/src/compress.c
index a2ede242..417ccc86 100644
--- a/src/compress.c
+++ b/src/compress.c
@@ -707,30 +707,47 @@ uncompressbuf(int fd, size_t bytes_max, size_t method, const unsigned char *old,
 		return makeerror(newch, n, "Cannot create pipe, %s",
 		    strerror(errno));
 	}
-	pid = fork();
+
+	/* For processes with large mapped virtual sizes, vfork
+	 * may be _much_ faster (10-100 times) than fork.
+	 */
+	pid = vfork();
 	if (pid < 0) {
-		return makeerror(newch, n, "Cannot fork, %s",
+		return makeerror(newch, n, "Cannot vfork, %s",
 		    strerror(errno));
 	}
 	if (pid == 0) {
 		/* child */
-
+		/* Note: we are after vfork, do not modify memory
+		 * in a way which confuses parent. In particular,
+		 * do not modify fdp[i][j].
+		 */
 		if (fd != -1) {
 			(void) lseek(fd, CAST(off_t, 0), SEEK_SET);
 			if (copydesc(STDIN_FILENO, fd))
-				close(fd);
+				(void) close(fd);
 		} else {
-			copydesc(STDIN_FILENO, fdp[STDIN_FILENO][0]); closep(fdp[STDIN_FILENO]);
+			if (copydesc(STDIN_FILENO, fdp[STDIN_FILENO][0]))
+				(void) close(fdp[STDIN_FILENO][0]);
+			if (fdp[STDIN_FILENO][1] > 2)
+				(void) close(fdp[STDIN_FILENO][1]);
 		}
-///FIXME: if one of the fdp[i][j] is 0, 1, or 2, this can bomb spectacularly
-		copydesc(STDOUT_FILENO, fdp[STDOUT_FILENO][1]); closep(fdp[STDOUT_FILENO]);
-		copydesc(STDERR_FILENO, fdp[STDERR_FILENO][1]); closep(fdp[STDERR_FILENO]);
+///FIXME: if one of the fdp[i][j] is 0 or 1, this can bomb spectacularly
+		if (copydesc(STDOUT_FILENO, fdp[STDOUT_FILENO][1]))
+			(void) close(fdp[STDOUT_FILENO][1]);
+		if (fdp[STDOUT_FILENO][0] > 2)
+			(void) close(fdp[STDOUT_FILENO][0]);
+
+		if (copydesc(STDERR_FILENO, fdp[STDERR_FILENO][1]))
+			(void) close(fdp[STDERR_FILENO][1]);
+		if (fdp[STDERR_FILENO][0] > 2)
+			(void) close(fdp[STDERR_FILENO][0]);
 
 		(void)execvp(compr[method].argv[0],
 		    RCAST(char *const *, RCAST(intptr_t, compr[method].argv)));
 		dprintf(STDERR_FILENO, "exec `%s' failed, %s",
 		    compr[method].argv[0], strerror(errno));
-		exit(1);
+		_exit(1); /* _exit(), not exit(), because of vfork */
 	}
 	/* parent */
 	/* Close write sides of child stdout/err pipes */
-- 
2.21.0