dpkg: Switch the filesdb module to use the FNV hash function

Use it instead of what seems to be a custom hash function. This seems
to reduce dispersion somewhat.

As a side effect this fixes an integer overflow.

Addresses: #760741

Warned-by: ASan
Reported-by: Bálint Réczey <balint@balintreczey.hu>

debian/changelog

@@ -111,6 +111,9 @@ dpkg (1.17.14) UNRELEASED; urgency=low
     Based on a patch by Bálint Réczey <balint@balintreczey.hu>. Closes: #760690
   * Switch the libdpkg string hashing function from FNV-1 to the recommended
     FNV-1a variant.
+  * Switch the dpkg files database string hashing function from what appears
+    to be a custom hash function to the libdpkg FNV-1a implementation. As a
+    side effect this fixes an integer overflow. Addresses: #760741
 
   [ Raphaël Hertzog ]
   * Explain better in deb-triggers(5) why interest/activate-noawait should be

src/filesdb.c

@@ -44,6 +44,7 @@
 #include <dpkg/i18n.h>
 #include <dpkg/dpkg.h>
 #include <dpkg/dpkg-db.h>
+#include <dpkg/string.h>
 #include <dpkg/path.h>
 #include <dpkg/dir.h>
 #include <dpkg/fdio.h>
@@ -523,9 +524,9 @@ struct fileiterator {
   int nbinn;
 };
 
-/* This must always be a power of two. If you change it consider changing
- * the per-character hashing factor (currently 1785 = 137 * 13) too. */
-#define BINS (1 << 17)
+/* This must always be a prime for optimal performance.
+ * This is the closest one to 2^17 (131072). */
+#define BINS 131071
 
 static struct filenamenode *bins[BINS];
 
@@ -576,12 +577,6 @@ void filesdbinit(void) {
     }
 }
 
-static int hash(const char *name) {
-  int v= 0;
-  while (*name) { v *= 1787; v += *name; name++; }
-  return v;
-}
-
 struct filenamenode *findnamenode(const char *name, enum fnnflags flags) {
   struct filenamenode **pointerp, *newnode;
   const char *orig_name = name;
@@ -590,7 +585,7 @@ struct filenamenode *findnamenode(const char *name, enum fnnflags flags) {
    * leading slash. */
   name = path_skip_slash_dotslash(name);
 
-  pointerp= bins + (hash(name) & (BINS-1));
+  pointerp = bins + (str_fnv_hash(name) % (BINS));
   while (*pointerp) {
     /* XXX: Why is the assert needed? It's checking already added entries. */
     assert((*pointerp)->name[0] == '/');