[PATCH] If NO_MMAP is defined, fake mmap() and munmap()

Since some platforms do not support mmap() at all, and others do only just
so, this patch introduces the option to fake mmap() and munmap() by
malloc()ing and read()ing explicitely.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>

compat/mmap.c

@@ -0,0 +1,113 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include "../cache.h"
+
+typedef struct fakemmapwritable {
+	void *start;
+	size_t length;
+	int fd;
+	off_t offset;
+	struct fakemmapwritable *next;
+} fakemmapwritable;
+
+static fakemmapwritable *writablelist = NULL;
+
+void *gitfakemmap(void *start, size_t length, int prot , int flags, int fd, off_t offset)
+{
+	int n = 0;
+
+	if(start != NULL)
+		die("Invalid usage of gitfakemmap.");
+
+	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);
+
+		if(count == 0) {
+			memset(start+n, 0, length-n);
+			break;
+		}
+
+		if(count < 0) {
+			free(start);
+			errno = EACCES;
+			return MAP_FAILED;
+		}
+
+		n += count;
+	}
+
+	if(prot & PROT_WRITE) {
+		fakemmapwritable *next = xmalloc(sizeof(fakemmapwritable));
+		next->start = start;
+		next->length = length;
+		next->fd = dup(fd);
+		next->offset = offset;
+		next->next = writablelist;
+		writablelist = next;
+	}
+
+	return start;
+}
+
+int gitfakemunmap(void *start, size_t length)
+{
+	fakemmapwritable *writable = writablelist, *before = NULL;
+
+	while(writable && (writable->start > start + length
+			|| writable->start + writable->length < start)) {
+		before = writable;
+		writable = writable->next;
+	}
+
+	if(writable) {
+		/* need to write back the contents */
+		int n = 0;
+
+		if(writable->start != start || writable->length != length)
+			die("fakemmap does not support partial write back.");
+
+		if(lseek(writable->fd, writable->offset, SEEK_SET) < 0) {
+			free(start);
+			errno = EBADF;
+			return -1;
+		}
+
+		while(n < length) {
+			int count = write(writable->fd, start + n, length - n);
+
+			if(count < 0) {
+				errno = EINVAL;
+				return -1;
+			}
+
+			n += count;
+		}
+
+		close(writable->fd);
+
+		if(before)
+			before->next = writable->next;
+		else
+			writablelist = writable->next;
+
+		free(writable);
+	}
+
+	free(start);
+
+	return 0;
+}
+