libfuzzer: fix SkipZigTest with new inputs

It is now ignored for inputs from the corpus to ensure the filesystem
and process corpus stay in sync. For other (nondeterministic) inputs the
input builder is now reset.

lib/fuzzer.zig

@@ -594,6 +594,21 @@ const Fuzzer = struct {
                 b.smithed_len = 4;
                 return input;
             }
+
+            pub fn reset(b: *Builder) void {
+                const uid_slices = b.uid_slices.entries.slice();
+                for (uid_slices.items(.key), uid_slices.items(.value)) |uid, *uid_data| {
+                    switch (uid.kind) {
+                        .int => uid_data.ints.deinit(gpa),
+                        .bytes => uid_data.bytes.deinit(gpa),
+                    }
+                }
+                b.uid_slices.clearRetainingCapacity();
+                b.total_ints = 0;
+                b.total_bytes = 0;
+                b.weighted_len = 0;
+                b.smithed_len = 4;
+            }
         };
     };
 
@@ -876,7 +891,14 @@ const Fuzzer = struct {
         // * The test has changed and a previous corpus input is being used
         // * An input provided by the test results in it
         // * The test is non-deterministic
-        if (f.runBytes(bytes, .bytes_fresh)) return;
+        if (f.runBytes(bytes, .bytes_fresh) and
+            modify_fs_corpus // The input is not from the filesystem.
+            // This is required to ensure the filesystem and process corpus are the same.
+        ) {
+            f.input_builder.reset();
+            f.corpus_pos = @enumFromInt(0);
+            return;
+        }
         f.req_values = f.input_builder.total_ints + f.input_builder.total_bytes;
         f.req_bytes = @intCast(f.input_builder.bytes_table.items.len);
         var input = f.input_builder.build();
@@ -1859,7 +1881,6 @@ const MemoryMappedInput = struct {
     /// Invalidates item pointers if more space is required.
     pub fn appendLittleInt(l: *MemoryMappedInput, T: type, x: T) void {
         l.ensureUnusedCapacity(@sizeOf(T));
-        //std.log.debug("{} {} {}", .{ l.writeSlice().len, l.len, @sizeOf(T) });
         l.writeSlice()[4 + l.len ..][0..@sizeOf(T)].* = @bitCast(mem.nativeToLittle(T, x));
         l.len += @sizeOf(T);
         l.writeLen();