reftable/reader: introduce refcounting

It was recently reported that concurrent reads and writes may cause the
reftable backend to segfault. The root cause of this is that we do not
properly keep track of reftable readers across reloads.

Suppose that you have a reftable iterator and then decide to reload the
stack while iterating through the iterator. When the stack has been
rewritten since we have created the iterator, then we would end up
discarding a subset of readers that may still be in use by the iterator.
The consequence is that we now try to reference deallocated memory,
which of course segfaults.

One way to trigger this is in t5616, where some background maintenance
jobs have been leaking from one test into another. This leads to stack
traces like the following one:

  + git -c protocol.version=0 -C pc1 fetch --filter=blob:limit=29999 --refetch origin
  AddressSanitizer:DEADLYSIGNAL
  =================================================================
  ==657994==ERROR: AddressSanitizer: SEGV on unknown address 0x7fa0f0ec6089 (pc 0x55f23e52ddf9 bp
0x7ffe7bfa1700 sp 0x7ffe7bfa1700 T0)
  ==657994==The signal is caused by a READ memory access.
      #0 0x55f23e52ddf9 in get_var_int reftable/record.c:29
      #1 0x55f23e53295e in reftable_decode_keylen reftable/record.c:170
      #2 0x55f23e532cc0 in reftable_decode_key reftable/record.c:194
      #3 0x55f23e54e72e in block_iter_next reftable/block.c:398
      #4 0x55f23e5573dc in table_iter_next_in_block reftable/reader.c:240
      #5 0x55f23e5573dc in table_iter_next reftable/reader.c:355
      #6 0x55f23e5573dc in table_iter_next reftable/reader.c:339
      #7 0x55f23e551283 in merged_iter_advance_subiter reftable/merged.c:69
      #8 0x55f23e55169e in merged_iter_next_entry reftable/merged.c:123
      #9 0x55f23e55169e in merged_iter_next_void reftable/merged.c:172
      #10 0x55f23e537625 in reftable_iterator_next_ref reftable/generic.c:175
      #11 0x55f23e2cf9c6 in reftable_ref_iterator_advance refs/reftable-backend.c:464
      #12 0x55f23e2d996e in ref_iterator_advance refs/iterator.c:13
      #13 0x55f23e2d996e in do_for_each_ref_iterator refs/iterator.c:452
      #14 0x55f23dca6767 in get_ref_map builtin/fetch.c:623
      #15 0x55f23dca6767 in do_fetch builtin/fetch.c:1659
      #16 0x55f23dca6767 in fetch_one builtin/fetch.c:2133
      #17 0x55f23dca6767 in cmd_fetch builtin/fetch.c:2432
      #18 0x55f23dba7764 in run_builtin git.c:484
      #19 0x55f23dba7764 in handle_builtin git.c:741
      #20 0x55f23dbab61e in run_argv git.c:805
      #21 0x55f23dbab61e in cmd_main git.c:1000
      #22 0x55f23dba4781 in main common-main.c:64
      #23 0x7fa0f063fc89 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
      #24 0x7fa0f063fd44 in __libc_start_main_impl ../csu/libc-start.c:360
      #25 0x55f23dba6ad0 in _start (git+0xadfad0) (BuildId: 803b2b7f59beb03d7849fb8294a8e2145dd4aa27)

While it is somewhat awkward that the maintenance processes survive
tests in the first place, it is totally expected that reftables should
work alright with concurrent writers. Seemingly they don't.

The only underlying resource that we need to care about in this context
is the reftable reader, which is responsible for reading a single table
from disk. These readers get discarded immediately (unless reused) when
calling `reftable_stack_reload()`, which is wrong. We can only close
them once we know that there are no iterators using them anymore.

Prepare for a fix by converting the reftable readers to be refcounted.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/reftable/reader.c b/reftable/reader.c
index 037417fcf63d4a0fd1ea3298a9f0220c46a7f67d..64a0953e687555e2142a3d4b7577eb32e189da8e 100644 (file)
--- a/reftable/reader.c
+++ b/reftable/reader.c
@@ -621,6 +621,7 @@ int reftable_reader_new(struct reftable_reader **out,
        r->source = *source;
        r->name = xstrdup(name);
        r->hash_id = 0;
+       r->refcount = 1;
 
        err = block_source_read_block(source, &footer, r->size,
                                      footer_size(r->version));
@@ -645,10 +646,21 @@ done:
        return err;
 }
 
-void reftable_reader_free(struct reftable_reader *r)
+void reftable_reader_incref(struct reftable_reader *r)
+{
+       if (!r->refcount)
+               BUG("cannot increment ref counter of dead reader");
+       r->refcount++;
+}
+
+void reftable_reader_decref(struct reftable_reader *r)
 {
        if (!r)
                return;
+       if (!r->refcount)
+               BUG("cannot decrement ref counter of dead reader");
+       if (--r->refcount)
+               return;
        block_source_close(&r->source);
        FREE_AND_NULL(r->name);
        reftable_free(r);
@@ -812,7 +824,7 @@ int reftable_reader_print_blocks(const char *tablename)
        }
 
 done:
-       reftable_reader_free(r);
+       reftable_reader_decref(r);
        table_iter_close(&ti);
        return err;
 }

diff --git a/reftable/reader.h b/reftable/reader.h
index 88b4f3b421299c14695b7f48c7a44a805d5919d1..3710ee09b4ca328a8ab990667b8ced8a98f6163f 100644 (file)
--- a/reftable/reader.h
+++ b/reftable/reader.h
@@ -50,6 +50,8 @@ struct reftable_reader {
        struct reftable_reader_offsets ref_offsets;
        struct reftable_reader_offsets obj_offsets;
        struct reftable_reader_offsets log_offsets;
+
+       uint64_t refcount;
 };
 
 const char *reader_name(struct reftable_reader *r);

diff --git a/reftable/readwrite_test.c b/reftable/readwrite_test.c
index 2f2ff787b261d2d0941ff1ce36ef304928965cd3..0494e7955a5f37c79026df4246ee1aedb310ea25 100644 (file)
--- a/reftable/readwrite_test.c
+++ b/reftable/readwrite_test.c
@@ -279,7 +279,7 @@ static void test_log_write_read(void)
        /* cleanup. */
        strbuf_release(&buf);
        free_names(names);
-       reftable_reader_free(reader);
+       reftable_reader_decref(reader);
 }
 
 static void test_log_zlib_corruption(void)
@@ -341,7 +341,7 @@ static void test_log_zlib_corruption(void)
        reftable_iterator_destroy(&it);
 
        /* cleanup. */
-       reftable_reader_free(reader);
+       reftable_reader_decref(reader);
        strbuf_release(&buf);
 }
 
@@ -383,7 +383,7 @@ static void test_table_read_write_sequential(void)
        EXPECT(j == N);
 
        reftable_iterator_destroy(&it);
-       reftable_reader_free(reader);
+       reftable_reader_decref(reader);
        strbuf_release(&buf);
        free_names(names);
 }
@@ -431,7 +431,7 @@ static void test_table_read_api(void)
        }
        reftable_iterator_destroy(&it);
        reftable_free(names);
-       reftable_reader_free(reader);
+       reftable_reader_decref(reader);
        strbuf_release(&buf);
 }
 
@@ -498,7 +498,7 @@ static void test_table_read_write_seek(int index, int hash_id)
                reftable_free(names[i]);
        }
        reftable_free(names);
-       reftable_reader_free(reader);
+       reftable_reader_decref(reader);
 }
 
 static void test_table_read_write_seek_linear(void)
@@ -611,7 +611,7 @@ static void test_table_refs_for(int indexed)
        strbuf_release(&buf);
        free_names(want_names);
        reftable_iterator_destroy(&it);
-       reftable_reader_free(reader);
+       reftable_reader_decref(reader);
 }
 
 static void test_table_refs_for_no_index(void)
@@ -657,7 +657,7 @@ static void test_write_empty_table(void)
        EXPECT(err > 0);
 
        reftable_iterator_destroy(&it);
-       reftable_reader_free(rd);
+       reftable_reader_decref(rd);
        strbuf_release(&buf);
 }
 
@@ -863,7 +863,7 @@ static void test_write_multiple_indices(void)
 
        reftable_iterator_destroy(&it);
        reftable_writer_free(writer);
-       reftable_reader_free(reader);
+       reftable_reader_decref(reader);
        strbuf_release(&writer_buf);
        strbuf_release(&buf);
 }
@@ -919,7 +919,7 @@ static void test_write_multi_level_index(void)
 
        reftable_iterator_destroy(&it);
        reftable_writer_free(writer);
-       reftable_reader_free(reader);
+       reftable_reader_decref(reader);
        strbuf_release(&writer_buf);
        strbuf_release(&buf);
 }

diff --git a/reftable/reftable-reader.h b/reftable/reftable-reader.h
index 8a05c846858a1b7f3dde6d5f02f52470bcbf4cbe..a600452b565803c25f502396910910cf97b9d8a0 100644 (file)
--- a/reftable/reftable-reader.h
+++ b/reftable/reftable-reader.h
@@ -33,6 +33,18 @@ struct reftable_reader;
 int reftable_reader_new(struct reftable_reader **pp,
                        struct reftable_block_source *src, const char *name);
 
+/*
+ * Manage the reference count of the reftable reader. A newly initialized
+ * reader starts with a refcount of 1 and will be deleted once the refcount has
+ * reached 0.
+ *
+ * This is required because readers may have longer lifetimes than the stack
+ * they belong to. The stack may for example be reloaded while the old tables
+ * are still being accessed by an iterator.
+ */
+void reftable_reader_incref(struct reftable_reader *reader);
+void reftable_reader_decref(struct reftable_reader *reader);
+
 /* Initialize a reftable iterator for reading refs. */
 void reftable_reader_init_ref_iterator(struct reftable_reader *r,
                                       struct reftable_iterator *it);
@@ -44,9 +56,6 @@ void reftable_reader_init_log_iterator(struct reftable_reader *r,
 /* returns the hash ID used in this table. */
 uint32_t reftable_reader_hash_id(struct reftable_reader *r);
 
-/* closes and deallocates a reader. */
-void reftable_reader_free(struct reftable_reader *);
-
 /* return an iterator for the refs pointing to `oid`. */
 int reftable_reader_refs_for(struct reftable_reader *r,
                             struct reftable_iterator *it, uint8_t *oid);

diff --git a/reftable/stack.c b/reftable/stack.c
index 0ac9cdf8de19fe846ad40ab7f30af66422f15398..8e85f8b4d99708507486a2a63a0607a012a0ee32 100644 (file)
--- a/reftable/stack.c
+++ b/reftable/stack.c
@@ -186,7 +186,7 @@ void reftable_stack_destroy(struct reftable_stack *st)
                        if (names && !has_name(names, name)) {
                                stack_filename(&filename, st, name);
                        }
-                       reftable_reader_free(st->readers[i]);
+                       reftable_reader_decref(st->readers[i]);
 
                        if (filename.len) {
                                /* On Windows, can only unlink after closing. */
@@ -290,7 +290,7 @@ static int reftable_stack_reload_once(struct reftable_stack *st,
                        const char *name = reader_name(cur[i]);
                        stack_filename(&table_path, st, name);
 
-                       reftable_reader_free(cur[i]);
+                       reftable_reader_decref(cur[i]);
 
                        /* On Windows, can only unlink after closing. */
                        unlink(table_path.buf);
@@ -299,7 +299,7 @@ static int reftable_stack_reload_once(struct reftable_stack *st,
 
 done:
        for (i = 0; i < new_readers_len; i++)
-               reftable_reader_free(new_readers[i]);
+               reftable_reader_decref(new_readers[i]);
        reftable_free(new_readers);
        reftable_free(cur);
        strbuf_release(&table_path);
@@ -1534,7 +1534,7 @@ static void remove_maybe_stale_table(struct reftable_stack *st, uint64_t max,
                goto done;
 
        update_idx = reftable_reader_max_update_index(rd);
-       reftable_reader_free(rd);
+       reftable_reader_decref(rd);
 
        if (update_idx <= max) {
                unlink(table_path.buf);

diff --git a/reftable/stack_test.c b/reftable/stack_test.c
index de0669b7b8a956109081e6f37a3fd70411ed2af8..bc3bf772749d545d9fca60a683d9864f58a9a961 100644 (file)
--- a/reftable/stack_test.c
+++ b/reftable/stack_test.c
@@ -1036,10 +1036,8 @@ static void test_reftable_stack_compaction_concurrent(void)
 static void unclean_stack_close(struct reftable_stack *st)
 {
        /* break abstraction boundary to simulate unclean shutdown. */
-       int i = 0;
-       for (; i < st->readers_len; i++) {
-               reftable_reader_free(st->readers[i]);
-       }
+       for (size_t i = 0; i < st->readers_len; i++)
+               reftable_reader_decref(st->readers[i]);
        st->readers_len = 0;
        FREE_AND_NULL(st->readers);
 }

diff --git a/t/helper/test-reftable.c b/t/helper/test-reftable.c
index 87c2f42aaed7903411cf121163819a16686f776f..f6d855a9d7ffae01de7eafa21cf4d878fcaf69bd 100644 (file)
--- a/t/helper/test-reftable.c
+++ b/t/helper/test-reftable.c
@@ -152,7 +152,7 @@ static int dump_reftable(const char *tablename)
 
 done:
        reftable_merged_table_free(mt);
-       reftable_reader_free(r);
+       reftable_reader_decref(r);
        return err;
 }
 

diff --git a/t/unit-tests/t-reftable-merged.c b/t/unit-tests/t-reftable-merged.c
index 8f51aca1b625abac987cc784a32ed22f262d90ba..081d3c8b69774a6a9254537428f7d19d474b72f7 100644 (file)
--- a/t/unit-tests/t-reftable-merged.c
+++ b/t/unit-tests/t-reftable-merged.c
@@ -115,7 +115,7 @@ merged_table_from_records(struct reftable_ref_record **refs,
 static void readers_destroy(struct reftable_reader **readers, const size_t n)
 {
        for (size_t i = 0; i < n; i++)
-               reftable_reader_free(readers[i]);
+               reftable_reader_decref(readers[i]);
        reftable_free(readers);
 }
 
@@ -437,7 +437,7 @@ static void t_default_write_opts(void)
        err = reftable_merged_table_new(&merged, &rd, 1, GIT_SHA1_FORMAT_ID);
        check(!err);
 
-       reftable_reader_free(rd);
+       reftable_reader_decref(rd);
        reftable_merged_table_free(merged);
        strbuf_release(&buf);
 }