journal-gatewayd: fix busy loop when following way beyond journal end

Fix regression introduced in a7bfb9f76b96888d60b4f287f29dcbf758ba34c0,
where busy loop can be started with a request for following logs with a
range header set with num_skip value pointing beyond the end of the
journal. In that case the reader callback returns 0 and is called
immediately again, usually causing an endless loop that is not recovered
even when new journal events are added.

The bug does not occur if num_skip is not set - in that case if no
journal entries matching the filters are added, the tight loop is
avoided by the sd_journal_wait().

To fix the issue, when no matching journal events are available, set a
flag and reuse the backoff mechanism using the sd_journal_wait().

Link: https://github.com/home-assistant/operating-system/issues/4190

diff --git a/src/journal-remote/journal-gatewayd.c b/src/journal-remote/journal-gatewayd.c
index 4d5743759967e163456e25dc6df7d5d92fc7188b..97acab82b70b8bf275a0340368a5279840782464 100644 (file)
--- a/src/journal-remote/journal-gatewayd.c
+++ b/src/journal-remote/journal-gatewayd.c
@@ -172,6 +172,7 @@ static ssize_t request_reader_entries(
 
         while (pos >= m->size) {
                 off_t sz;
+                bool wait_for_events = false;
 
                 /* End of this entry, so let's serialize the next
                  * one */
@@ -192,9 +193,10 @@ static ssize_t request_reader_entries(
                          * from it are not returned. */
                         if (r < m->n_skip + 1) {
                                 m->n_skip -= r;
-                                if (m->follow)
-                                        return 0;
-                                return MHD_CONTENT_READER_END_OF_STREAM;
+
+                                if (!m->follow)
+                                        return MHD_CONTENT_READER_END_OF_STREAM;
+                                wait_for_events = true;
                         }
                 } else
                         r = sd_journal_next(m->journal);
@@ -203,20 +205,21 @@ static ssize_t request_reader_entries(
                         log_error_errno(r, "Failed to advance journal pointer: %m");
                         return MHD_CONTENT_READER_END_WITH_ERROR;
                 } else if (r == 0) {
+                        if (!m->follow)
+                                return MHD_CONTENT_READER_END_OF_STREAM;
+                        wait_for_events = true;
+                }
 
-                        if (m->follow) {
-                                r = sd_journal_wait(m->journal, (uint64_t) JOURNAL_WAIT_TIMEOUT);
-                                if (r < 0) {
-                                        log_error_errno(r, "Couldn't wait for journal event: %m");
-                                        return MHD_CONTENT_READER_END_WITH_ERROR;
-                                }
-                                if (r == SD_JOURNAL_NOP)
-                                        break;
-
-                                continue;
+                if (wait_for_events) {
+                        r = sd_journal_wait(m->journal, (uint64_t) JOURNAL_WAIT_TIMEOUT);
+                        if (r < 0) {
+                                log_error_errno(r, "Couldn't wait for journal event: %m");
+                                return MHD_CONTENT_READER_END_WITH_ERROR;
                         }
+                        if (r == SD_JOURNAL_NOP)
+                                break;
 
-                        return MHD_CONTENT_READER_END_OF_STREAM;
+                        continue;
                 }
 
                 if (m->discrete) {