querylog: the aside file's name says why, so a schema change is not called corrupt

src/storage/querylog_schema.zig

@@ -138,7 +138,7 @@ pub fn open(io: std.Io, dir: std.Io.Dir, path: [:0]const u8) Error!OpenResult {
     const aside: ?[]const u8 = if (cause == .missing)
         null
     else
-        try renameAside(io, dir, path, &aside_buf);
+        try renameAside(io, dir, path, cause, &aside_buf);
 
     // Not optional: a stale WAL left beside the renamed database would be
     // replayed into the freshly created file and corrupt it immediately. Any
@@ -178,20 +178,37 @@ fn quickCheck(database: *db.Db) db.Error!bool {
     return std.ascii.eqlIgnoreCase(stmt.columnText(0), "ok");
 }
 
+/// What the aside file's name calls the reason it was set aside.
+///
+/// The name is the only account of the reason an operator gets: the log line
+/// naming it scrolls away, the file stays for months. `fingerprint_mismatch` is
+/// a database with nothing wrong with it — this build's DDL moved — so calling
+/// its file "corrupt" invites the operator to delete evidence of a healthy file.
+fn asideTag(reason: RecreateReason) []const u8 {
+    return switch (reason) {
+        .missing => unreachable, // there is no file to rename
+        .corrupt => "corrupt",
+        .not_a_database => "not-a-database",
+        .quick_check_failed => "quick-check-failed",
+        .fingerprint_mismatch => "schema-changed",
+    };
+}
+
 /// Renames the unusable file out of the way and returns the name it now has.
 ///
 /// `renamePreserve` is `RENAME_NOREPLACE`: it returns `error.PathAlreadyExists`
-/// instead of overwriting. A previously saved corrupt file must never be
-/// destroyed by the next recreate, and two recreates in the same second are not
-/// hypothetical on a boot loop — hence the uniquifying retries.
-fn renameAside(io: std.Io, dir: std.Io.Dir, path: []const u8, buf: []u8) Error![]const u8 {
+/// instead of overwriting. A previously saved file must never be destroyed by
+/// the next recreate, and two recreates in the same second are not hypothetical
+/// on a boot loop — hence the uniquifying retries.
+fn renameAside(io: std.Io, dir: std.Io.Dir, path: []const u8, reason: RecreateReason, buf: []u8) Error![]const u8 {
+    const tag = asideTag(reason);
     const seconds = std.Io.Clock.real.now(io).toSeconds();
     var attempt: u32 = 0;
     while (attempt < 100) : (attempt += 1) {
         const aside = if (attempt == 0)
-            std.fmt.bufPrint(buf, "{s}.corrupt-{d}", .{ path, seconds }) catch return error.NameTooLong
+            std.fmt.bufPrint(buf, "{s}.{s}-{d}", .{ path, tag, seconds }) catch return error.NameTooLong
         else
-            std.fmt.bufPrint(buf, "{s}.corrupt-{d}-{d}", .{ path, seconds, attempt }) catch return error.NameTooLong;
+            std.fmt.bufPrint(buf, "{s}.{s}-{d}-{d}", .{ path, tag, seconds, attempt }) catch return error.NameTooLong;
 
         dir.renamePreserve(path, dir, aside, io) catch |e| switch (e) {
             error.PathAlreadyExists => continue,
@@ -287,6 +304,13 @@ test "recreatable is a whitelist and never selects a resource error" {
     }
 }
 
+test "the aside name says why, and a healthy file is never called corrupt" {
+    try testing.expectEqualStrings("corrupt", asideTag(.corrupt));
+    try testing.expectEqualStrings("not-a-database", asideTag(.not_a_database));
+    try testing.expectEqualStrings("quick-check-failed", asideTag(.quick_check_failed));
+    try testing.expectEqualStrings("schema-changed", asideTag(.fingerprint_mismatch));
+}
+
 test "recreatable selects exactly two of db.Error's members" {
     // Exhaustive over the whole set, so a variant added to `db.Error` later
     // defaults to propagate. The list above only proves the named errors are