imapserver: implement "inprogress" response code (RFC 9585) for keepalive during long search

For long searches in big mailboxes, without any matches, we would previously
keep working and not say anything. Clients could interpret this silence as a
broken connection at some point. We now send a "we're still searching" untagged
OK responses with code INPROGRESS every 10 seconds while we're still searching,
to prevent the client from closing the connection. We also send how many
messages we've processed, and usually also how many we need to process in grand
total. Clients can use this to show a progress bar.

imapclient/parse.go

@@ -131,6 +131,7 @@ var knownCodes = stringMap(
 	// With parameters.
 	"BADCHARSET", "CAPABILITY", "PERMANENTFLAGS", "UIDNEXT", "UIDVALIDITY", "UNSEEN", "APPENDUID", "COPYUID",
 	"HIGHESTMODSEQ", "MODIFIED",
+	"INPROGRESS", // ../rfc/9585:104
 )
 
 func stringMap(l ...string) map[string]struct{} {
@@ -222,6 +223,30 @@ func (c *Conn) xrespCode() (string, CodeArg) {
 		c.xspace()
 		modified := c.xuidset()
 		codeArg = CodeModified(NumSet{Ranges: modified})
+	case "INPROGRESS":
+		// ../rfc/9585:238
+		var tag string
+		var current, goal *uint32
+		if c.space() {
+			c.xtake("(")
+			tag = c.xquoted()
+			c.xspace()
+			if c.peek('n') || c.peek('N') {
+				c.xtake("nil")
+			} else {
+				v := c.xuint32()
+				current = &v
+			}
+			c.xspace()
+			if c.peek('n') || c.peek('N') {
+				c.xtake("nil")
+			} else {
+				v := c.xnzuint32()
+				goal = &v
+			}
+			c.xtake(")")
+		}
+		codeArg = CodeInProgress{tag, current, goal}
 	}
 	return W, codeArg
 }

imapclient/protocol.go

@@ -159,6 +159,32 @@ func (c CodeHighestModSeq) CodeString() string {
 	return fmt.Sprintf("HIGHESTMODSEQ %d", c)
 }
 
+// "INPROGRESS" response code.
+type CodeInProgress struct {
+	Tag     string // Nil is empty string.
+	Current *uint32
+	Goal    *uint32
+}
+
+func (c CodeInProgress) CodeString() string {
+	// ABNF allows inprogress-tag/state with all nil values. Doesn't seem useful enough
+	// to keep track of.
+	if c.Tag == "" && c.Current == nil && c.Goal == nil {
+		return "INPROGRESS"
+	}
+
+	// todo: quote tag properly
+	current := "nil"
+	goal := "nil"
+	if c.Current != nil {
+		current = fmt.Sprintf("%d", *c.Current)
+	}
+	if c.Goal != nil {
+		goal = fmt.Sprintf("%d", *c.Goal)
+	}
+	return fmt.Sprintf("INPROGRESS (%q %s %s)", c.Tag, current, goal)
+}
+
 // RespText represents a response line minus the leading tag.
 type RespText struct {
 	Code    string  // The first word between [] after the status.