content-files: call open() with O_TRUNC

Problem: In content-files, most files are created using a hash
of the data.  So in most cases the filenames are unique.  However,
the kvs-checkpoint.put rpc will typically write to the same file
over and over again.  In the event the file already exists, content-files
did not clear the data in a file before writing.  This can lead to
corrupted data if the user wrote fewer bytes to the file than before.

Solution: Call open() with O_TRUNC to truncate the file before writing
new data.  Add tests increasing / decreasing checkpoint data size.

src/modules/content-files/filedb.c

@@ -87,7 +87,7 @@ int filedb_put (const char *dbpath,
             *errstr = "key name too long for internal buffer";
         return -1;
     }
-    if ((fd = open (path, O_WRONLY | O_CREAT, 0666)) < 0)
+    if ((fd = open (path, O_WRONLY | O_CREAT | O_TRUNC, 0666)) < 0)
         return -1;
     if (write_all (fd, data, size) < 0) {
         ERRNO_SAFE_WRAP (close, fd);

t/t0018-content-files.t

@@ -156,6 +156,26 @@ test_expect_success HAVE_JQ 'kvs-checkpoint.get foo still returns rootref baz' '
         test_cmp rootref3.exp rootref3.out
 '
 
+test_expect_success HAVE_JQ 'kvs-checkpoint.put updates foo rooref with longer rootref' '
+        kvs_checkpoint_put foo abcdefghijklmnopqrstuvwxyz
+'
+
+test_expect_success HAVE_JQ 'kvs-checkpoint.get foo returned rootref with longer rootref' '
+        echo abcdefghijklmnopqrstuvwxyz >rootref3.exp &&
+        kvs_checkpoint_get foo | jq -r .value | jq -r .rootref >rootref3.out &&
+        test_cmp rootref3.exp rootref3.out
+'
+
+test_expect_success HAVE_JQ 'kvs-checkpoint.put updates foo rooref to shorter rootref' '
+        kvs_checkpoint_put foo foobar
+'
+
+test_expect_success HAVE_JQ 'kvs-checkpoint.get foo returned rootref with shorter rootref' '
+        echo foobar >rootref4.exp &&
+        kvs_checkpoint_get foo | jq -r .value | jq -r .rootref >rootref4.out &&
+        test_cmp rootref4.exp rootref4.out
+'
+
 test_expect_success 'load with invalid blobref fails' '
 	test_must_fail backing_load notblobref 2>notblobref.err &&
 	grep "invalid blobref" notblobref.err