Replacing the system call pread() with lseek()/xread()/lseek() sequence.

Using cygwin with cygwin.dll before 1.5.22 the system call pread() is buggy.
This patch introduces NO_PREAD. If NO_PREAD is set git uses a sequence of
lseek()/xread()/lseek() to emulate pread.

Signed-off-by: Stefan-W. Hahn <stefan.hahn@s-hahn.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Makefile

@@ -69,6 +69,9 @@ all:
 #
 # Define NO_MMAP if you want to avoid mmap.
 #
+# Define NO_PREAD if you have a problem with pread() system call (e.g.
+# cygwin.dll before v1.5.22).
+#
 # Define NO_FAST_WORKING_DIRECTORY if accessing objects in pack files is
 # generally faster on your platform than accessing the working directory.
 #
@@ -523,6 +526,10 @@ ifdef NO_MMAP
 	COMPAT_CFLAGS += -DNO_MMAP
 	COMPAT_OBJS += compat/mmap.o
 endif
+ifdef NO_PREAD
+	COMPAT_CFLAGS += -DNO_PREAD
+	COMPAT_OBJS += compat/pread.o
+endif
 ifdef NO_FAST_WORKING_DIRECTORY
 	BASIC_CFLAGS += -DNO_FAST_WORKING_DIRECTORY
 endif

compat/pread.c

@@ -0,0 +1,18 @@
+#include "../git-compat-util.h"
+
+ssize_t git_pread(int fd, void *buf, size_t count, off_t offset)
+{
+        off_t current_offset;
+        ssize_t rc;
+
+        current_offset = lseek(fd, 0, SEEK_CUR);
+
+        if (lseek(fd, offset, SEEK_SET) < 0)
+                return -1;
+
+        rc = read_in_full(fd, buf, count);
+
+        if (current_offset != lseek(fd, current_offset, SEEK_SET))
+                return -1;
+        return rc;
+}

git-compat-util.h

@@ -107,6 +107,11 @@ extern int git_munmap(void *start, size_t length);
 #define DEFAULT_PACKED_GIT_LIMIT \
 	((1024L * 1024L) * (sizeof(void*) >= 8 ? 8192 : 256))
 
+#ifdef NO_PREAD
+#define pread git_pread
+extern ssize_t git_pread(int fd, void *buf, size_t count, off_t offset);
+#endif
+
 #ifdef NO_SETENV
 #define setenv gitsetenv
 extern int gitsetenv(const char *, const char *, int);