Add compat/fopen.c which returns NULL on attempt to open directory

Some systems do not fail as expected when fread et al. are called on
a directory stream. Replace fopen on such systems which will fail
when the supplied path is a directory.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -3,6 +3,9 @@ all::
 
 # Define V=1 to have a more verbose compile.
 #
+# Define FREAD_READS_DIRECTORIES if your are on a system which succeeds
+# when attempting to read from an fopen'ed directory.
+#
 # Define NO_OPENSSL environment variable if you do not have OpenSSL.
 # This also implies MOZILLA_SHA1.
 #
@@ -618,6 +621,10 @@ endif
 ifdef NO_C99_FORMAT
 	BASIC_CFLAGS += -DNO_C99_FORMAT
 endif
+ifdef FREAD_READS_DIRECTORIES
+	COMPAT_CFLAGS += -DFREAD_READS_DIRECTORIES
+	COMPAT_OBJS += compat/fopen.o
+endif
 ifdef NO_SYMLINK_HEAD
 	BASIC_CFLAGS += -DNO_SYMLINK_HEAD
 endif

compat/fopen.c

@@ -0,0 +1,26 @@
+#include "../git-compat-util.h"
+#undef fopen
+FILE *git_fopen(const char *path, const char *mode)
+{
+	FILE *fp;
+	struct stat st;
+
+	if (mode[0] == 'w' || mode[0] == 'a')
+		return fopen(path, mode);
+
+	if (!(fp = fopen(path, mode)))
+		return NULL;
+
+	if (fstat(fileno(fp), &st)) {
+		fclose(fp);
+		return NULL;
+	}
+
+	if (S_ISDIR(st.st_mode)) {
+		fclose(fp);
+		errno = EISDIR;
+		return NULL;
+	}
+
+	return fp;
+}

git-compat-util.h

@@ -204,6 +204,11 @@ void *gitmemmem(const void *haystack, size_t haystacklen,
                 const void *needle, size_t needlelen);
 #endif
 
+#ifdef FREAD_READS_DIRECTORIES
+#define fopen(a,b) git_fopen(a,b)
+extern FILE *git_fopen(const char*, const char*);
+#endif
+
 #ifdef __GLIBC_PREREQ
 #if __GLIBC_PREREQ(2, 1)
 #define HAVE_STRCHRNUL