Correctly handle direct mode hit, missing object file and differing cpp hash

ccache.c

@@ -1402,6 +1402,7 @@ static void ccache(int argc, char *argv[])
 	time_t t;
 	struct tm *tm;
 	int put_object_in_manifest = 0;
+	struct file_hash *object_hash_from_manifest = NULL;
 
 	t = time(NULL);
 	tm = localtime(&t);
@@ -1469,6 +1470,9 @@ static void ccache(int argc, char *argv[])
 			 * so don't readd it later.
 			 */
 			put_object_in_manifest = 0;
+
+			object_hash_from_manifest = object_hash;
+			object_hash = NULL;
 		} else {
 			/* Add object to manifest later. */
 			put_object_in_manifest = 1;
@@ -1481,6 +1485,31 @@ static void ccache(int argc, char *argv[])
 	 */
 	find_hash(stripped_args, FINDHASH_CPP_MODE);
 
+	if (object_hash_from_manifest
+	    && !file_hashes_equal(object_hash_from_manifest, object_hash)) {
+		/*
+		 * The hash from manifest differs from the hash of the
+		 * preprocessor output. This could be because:
+		 *
+		 * - The preprocessor produces different output for the same
+		 *   input (not likely).
+		 * - There's a bug in ccache (maybe incorrect handling of
+		 *   compiler arguments).
+		 * - The user has used a different CCACHE_BASEDIR (most
+		 *   likely).
+		 *
+		 * The best thing here would probably be to remove the hash
+		 * entry from the manifest. For now, we use a simpler method:
+		 * just remove the manifest file.
+		 */
+		cc_log("Hash from manifest doesn't match preprocessor output\n");
+		cc_log("Likely reason: different CCACHE_BASEDIRs used\n");
+		cc_log("Removing manifest as a safety measure\n");
+		unlink(manifest_path);
+
+		put_object_in_manifest = 1;
+	}
+
 	/* if we can return from cache at this point then do */
 	from_cache(FROMCACHE_CPP_MODE, put_object_in_manifest);
 

hashutil.c

@@ -12,3 +12,9 @@ int strings_equal(void *str1, void *str2)
 {
 	return strcmp((const char *)str1, (const char *)str2) == 0;
 }
+
+int file_hashes_equal(struct file_hash *fh1, struct file_hash *fh2)
+{
+	return memcmp(fh1->hash, fh2->hash, 16) == 0
+		&& fh1->size == fh2->size;
+}

hashutil.h

@@ -1,7 +1,16 @@
 #ifndef HASHUTIL_H
 #define HASHUTIL_H
 
+#include <inttypes.h>
+
+struct file_hash
+{
+	uint8_t hash[16];
+	uint32_t size;
+};
+
 unsigned int hash_from_string(void *str);
 int strings_equal(void *str1, void *str2);
+int file_hashes_equal(struct file_hash *fh1, struct file_hash *fh2);
 
 #endif

manifest.h

@@ -1,15 +1,9 @@
 #ifndef MANIFEST_H
 #define MANIFEST_H
 
-#include <inttypes.h>
+#include "hashutil.h"
 #include "hashtable.h"
 
-struct file_hash
-{
-	uint8_t hash[16];
-	uint32_t size;
-};
-
 struct file_hash *manifest_get(const char *manifest_path);
 int manifest_put(const char *manifest_path, struct file_hash *object_hash,
                  struct hashtable *included_files);