[geary: 2/3] Fix bug with message retention count during GC plus fix test case

[Michael Gratton <mjog src gnome org>]

commit 4fdaaeacd9720d4b5b2d48be76c316a6a824c42f
Author: Chris Heywood <15127-creywood users noreply gitlab gnome org>
Date:   Mon Aug 24 19:29:44 2020 +1000

    Fix bug with message retention count during GC plus fix test case

 src/engine/imap-db/imap-db-folder.vala       | 27 +++++++++++++++++++++++----
 test/engine/imap-db/imap-db-folder-test.vala | 25 ++++++++++++++++++++++++-
 2 files changed, 47 insertions(+), 5 deletions(-)
---
diff --git a/src/engine/imap-db/imap-db-folder.vala b/src/engine/imap-db/imap-db-folder.vala
index 52f3e82fb..de015f5a5 100644
--- a/src/engine/imap-db/imap-db-folder.vala
+++ b/src/engine/imap-db/imap-db-folder.vala
@@ -946,24 +946,43 @@ private class Geary.ImapDB.Folder : BaseObject, Geary.ReferenceSemantics {
             // MessageLocationTable.ordering isn't relied on due to IMAP folder
             // UIDs not guaranteed to be in order.
             StringBuilder sql = new StringBuilder();
+            sql.append("""
+                SELECT COUNT(*)
+                FROM MessageLocationTable ml
+                INNER JOIN MessageTable m
+                INDEXED BY MessageTableInternalDateTimeTIndex
+                    ON ml.message_id = m.id
+                WHERE ml.folder_id = ?
+                AND m.internaldate_time_t >= ?
+            """);
+
+            Db.Statement stmt = cx.prepare(sql.str);
+            stmt.bind_rowid(0, folder_id);
+            stmt.bind_int64(1, cutoff.to_unix());
+            Db.Result results = stmt.exec(cancellable);
+            int64 found_within_threshold = results.int64_at(0);
+            int64 extra_to_retain = 
+                (MINIMUM_MESSAGES_TO_RETAIN_DURING_GC - found_within_threshold).clamp(0, int64.MAX);
+
+            sql = new StringBuilder();
             sql.append("""
                 SELECT ml.id, ml.message_id, ml.ordering
                 FROM MessageLocationTable ml
                 INNER JOIN MessageTable m
                 INDEXED BY MessageTableInternalDateTimeTIndex
                     ON ml.message_id = m.id
-                WHERE folder_id = ?
+                WHERE ml.folder_id = ?
                 AND m.internaldate_time_t < ?
                 ORDER BY m.internaldate_time_t DESC
                 LIMIT -1 OFFSET ?;
             """);
 
-            Db.Statement stmt = cx.prepare(sql.str);
+            stmt = cx.prepare(sql.str);
             stmt.bind_rowid(0, folder_id);
             stmt.bind_int64(1, cutoff.to_unix());
-            stmt.bind_int64(2, MINIMUM_MESSAGES_TO_RETAIN_DURING_GC);
+            stmt.bind_int64(2, extra_to_retain);
 
-            Db.Result results = stmt.exec(cancellable);
+            results = stmt.exec(cancellable);
 
             while (!results.finished) {
                 if (deleted_email_ids == null) {
diff --git a/test/engine/imap-db/imap-db-folder-test.vala b/test/engine/imap-db/imap-db-folder-test.vala
index be02bcb5e..bce1fb43c 100644
--- a/test/engine/imap-db/imap-db-folder-test.vala
+++ b/test/engine/imap-db/imap-db-folder-test.vala
@@ -366,6 +366,22 @@ class Geary.ImapDB.FolderTest : TestCase {
                 (3, 3, 1, 2, 1);
         """);
 
+        for (int i = 4; i <= 200; i++) {
+            this.account.db.exec(
+                "INSERT INTO MessageTable (id, fields, to_field, internaldate_time_t) " +
+                "VALUES (%d, %d, '%s', %s);".printf(i,
+                                                fixture_fields,
+                                                fixture_to,
+                                                beyond_threshold.to_unix().to_string())
+            );
+            this.account.db.exec(
+                "INSERT INTO MessageLocationTable " + 
+                "    (id, message_id, folder_id, ordering, remove_marker) " + 
+                "VALUES (%d, %d, 1, %d, 1);".printf(i, i, i)
+            );
+        }
+
+
         this.folder.detach_emails_before_timestamp.begin(
             threshold,
             null,
@@ -375,7 +391,7 @@ class Geary.ImapDB.FolderTest : TestCase {
 
         int64[] expected = { 1, 2 };
         Db.Result result = this.account.db.query(
-            "SELECT id FROM MessageLocationTable"
+            "SELECT id FROM MessageLocationTable WHERE id IN (1, 2);"
         );
 
         int i = 0;
@@ -386,6 +402,13 @@ class Geary.ImapDB.FolderTest : TestCase {
             result.next();
         }
         assert_true(i == expected.length, "Not enough rows");
+
+        result = this.account.db.query(
+            "SELECT COUNT(id) FROM MessageLocationTable WHERE folder_id = 1;"
+        );
+        assert_false(result.finished);
+        assert_equal<int64?>(result.int64_at(0), 100);
+        result.next();
     }
 
     private Email new_mock_remote_email(int64 uid,