[geary/wip/imap-db-acccount-test-flakey] Don't reccommend db garbage collection if there are no messages

[Michael Gratton <mjog src gnome org>]

commit fe1d65c286d33bcdf1106885c8e4b16e2383620b
Author: Michael Gratton <mike vee net>
Date:   Tue Jan 22 10:34:39 2019 +1100

    Don't reccommend db garbage collection if there are no messages
    
    This prevents the GC running during (most) unit tests, which might also
    be causing intermittent hangs in ImapDB.AccountTest and DatabaseTest.

 src/engine/imap-db/imap-db-gc.vala | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)
---
diff --git a/src/engine/imap-db/imap-db-gc.vala b/src/engine/imap-db/imap-db-gc.vala
index ce52e990..5f21bcbe 100644
--- a/src/engine/imap-db/imap-db-gc.vala
+++ b/src/engine/imap-db/imap-db-gc.vala
@@ -107,11 +107,15 @@ private class Geary.ImapDB.GC {
             (last_vacuum_time != null) ? last_vacuum_time.to_string() : "never",
             reaped_messages_since_last_vacuum,
             free_page_bytes.to_string());
-        
+
         RecommendedOperation op = RecommendedOperation.NONE;
-        DateTime now = new DateTime.now_local();
-        
+        if (!yield has_message_rows(cancellable)) {
+            // No message rows exist, so don't bother vacuuming
+            return op;
+        }
+
         // Reap every REAP_DAYS_SPAN unless never executed, in which case run now
+        DateTime now = new DateTime.now_local();
         int64 days;
         if (last_reap_time == null) {
             // null means reaping has never executed
@@ -613,7 +617,24 @@ private class Geary.ImapDB.GC {
         
         return deleted;
     }
-    
+
+    private async bool has_message_rows(GLib.Cancellable? cancellable) {
+        bool ret = false;
+        yield db.exec_transaction_async(Db.TransactionType.RO, (cx) => {
+                Db.Result result = cx.query(
+                    "SELECT count(*) FROM MessageTable LIMIT 1"
+                );
+
+                Db.TransactionOutcome txn_ret = FAILURE;
+                if (!result.finished) {
+                    txn_ret = SUCCESS;
+                    ret = result.int64_at(0) > 0;
+                }
+                return txn_ret;
+            }, cancellable);
+        return ret;
+    }
+
     private async void fetch_gc_info_async(Cancellable? cancellable, out DateTime? last_reap_time,
         out DateTime? last_vacuum_time, out int reaped_messages_since_last_vacuum, out int64 free_page_bytes)
         throws Error {