handle case-insensitivity of IMAP's INBOX

this is relevant only when listing an IMAP Store's contents, as that's
the only place where we aren't imposing the spelling ourselves.

we need to be careful not to treat our own canonical (prefix-stripped
and always slash-delimited) box names like that; codify that in
comments.

this reveals that commit 6f2160f1 may be deemed to have been incorrect -
the TODO item was ambiguous, and could quite possibly have meant this
fix. unsurprisingly, 380ccdd4 re-introduced it with more explicit
wording.

TODO

@@ -37,8 +37,6 @@ Patterns.
   function being missing so far
 - this is needed for move detection, which would work only within one Channel
 
-normalize INBOX capitalization received from IMAP, to avoid anomalies.
-
 kill the concept of an INBOX, it is a relic from single-channel operation.
 if somebody needs it, he can have two stores with different Paths. the path
 can name a single (in-)box (curr. broken with maildir). an empty box name

src/drv_imap.c

@@ -1232,8 +1232,20 @@ parse_list_rsp_p1( imap_store_t *ctx, list_t *list, char *cmd ATTR_UNUSED )
 	return parse_list( ctx, cmd, parse_list_rsp_p2 );
 }
 
+// Use this to check whether a full path refers to the actual IMAP INBOX.
 static int
 is_inbox( imap_store_t *ctx, const char *arg, int argl )
+{
+	if (!starts_with_upper( arg, argl, "INBOX", 5 ))
+		return 0;
+	if (arg[5] && arg[5] != ctx->delimiter[0])
+		return 0;
+	return 1;
+}
+
+// Use this to check whether a path fragment collides with the canonical INBOX.
+static int
+is_INBOX( imap_store_t *ctx, const char *arg, int argl )
 {
 	if (!starts_with( arg, argl, "INBOX", 5 ))
 		return 0;
@@ -1256,16 +1268,22 @@ parse_list_rsp_p2( imap_store_t *ctx, list_t *list, char *cmd ATTR_UNUSED )
 	}
 	arg = list->val;
 	argl = list->len;
-	if ((l = strlen( ctx->prefix ))) {
-		if (starts_with( arg, argl, ctx->prefix, l )) {
-			arg += l;
-			argl -= l;
-			if (is_inbox( ctx, arg, argl )) {
-				if (!arg[5])
-					warn( "IMAP warning: ignoring INBOX in %s\n", ctx->prefix );
-				goto skip;
-			}
-		} else if (!is_inbox( ctx, arg, argl )) {
+	if (is_inbox( ctx, arg, argl )) {
+		// The server might be weird and have a non-uppercase INBOX. It
+		// may legitimately do so, but we need the canonical spelling.
+		memcpy( arg, "INBOX", 5 );
+	} else if ((l = strlen( ctx->prefix ))) {
+		if (!starts_with( arg, argl, ctx->prefix, l ))
+			goto skip;
+		arg += l;
+		argl -= l;
+		// A folder named "INBOX" would be indistinguishable from the
+		// actual INBOX after prefix stripping, so drop it. This applies
+		// only to the fully uppercased spelling, as our canonical box
+		// names are case-sensitive (unlike IMAP's INBOX).
+		if (is_INBOX( ctx, arg, argl )) {
+			if (!arg[5])  // No need to complain about subfolders as well.
+				warn( "IMAP warning: ignoring INBOX in %s\n", ctx->prefix );
 			goto skip;
 		}
 	}

src/drv_maildir.c

@@ -352,6 +352,7 @@ maildir_list_maildirpp( maildir_store_t *ctx, int flags, const char *inbox )
 		} else {
 			if (!(flags & (LIST_PATH | LIST_PATH_MAYBE)))
 				continue;
+			// Explained in maildir_list_recurse().
 			if (starts_with( ent, -1, "INBOX", 5 ) && (!ent[5] || ent[5] == '.')) {
 				if (!warned) {
 					warned = 1;
@@ -437,6 +438,10 @@ maildir_list_recurse( maildir_store_t *ctx, int isBox, int flags,
 						continue;
 				}
 			}
+			// A folder named "INBOX" would be indistinguishable from the
+			// actual INBOX after prefix stripping, so drop it. This applies
+			// only to the fully uppercased spelling, as our canonical box
+			// names are case-sensitive (unlike IMAP's INBOX).
 			if (!nameLen && equals( ent, -1, "INBOX", 5 )) {
 				path[pathLen] = 0;
 				warn( "Maildir warning: ignoring INBOX in %s\n", path );