Switch git_mmap to use pread.

Now that Git depends on pread in index-pack its safe to say we can
also depend on it within the git_mmap emulation we activate when
NO_MMAP is set.  On most systems pread should be slightly faster
than an lseek/read/lseek sequence as its one system call vs. three
system calls.

We also now honor EAGAIN and EINTR error codes from pread and
restart the prior read.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff --git a/compat/mmap.c b/compat/mmap.c
index bb34c7e95f6aafd6b8a433db0b2ceb0a68086817..4cfaee31361d98d2c8f68d250609b09130335baa 100644 (file)
--- a/compat/mmap.c
+++ b/compat/mmap.c
@@ -2,17 +2,11 @@
 
 void *git_mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset)
 {
-       int n = 0;
-       off_t current_offset = lseek(fd, 0, SEEK_CUR);
+       size_t n = 0;
 
        if (start != NULL || !(flags & MAP_PRIVATE))
                die("Invalid usage of mmap when built with NO_MMAP");
 
-       if (lseek(fd, offset, SEEK_SET) < 0) {
-               errno = EINVAL;
-               return MAP_FAILED;
-       }
-
        start = xmalloc(length);
        if (start == NULL) {
                errno = ENOMEM;
@@ -20,14 +14,16 @@ void *git_mmap(void *start, size_t length, int prot, int flags, int fd, off_t of
        }
 
        while (n < length) {
-               int count = read(fd, start+n, length-n);
+               ssize_t count = pread(fd, (char *)start + n, length - n, offset + n);
 
                if (count == 0) {
-                       memset(start+n, 0, length-n);
+                       memset((char *)start+n, 0, length-n);
                        break;
                }
 
                if (count < 0) {
+                       if (errno == EAGAIN || errno == EINTR)
+                               continue;
                        free(start);
                        errno = EACCES;
                        return MAP_FAILED;
@@ -36,11 +32,6 @@ void *git_mmap(void *start, size_t length, int prot, int flags, int fd, off_t of
                n += count;
        }
 
-       if (current_offset != lseek(fd, current_offset, SEEK_SET)) {
-               errno = EINVAL;
-               return MAP_FAILED;
-       }
-
        return start;
 }