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>

compat/mmap.c

@@ -2,32 +2,28 @@
 
 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;
 		return MAP_FAILED;
 	}
 
 	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;
 }