make socket_read()'s interface more like socket_read_line()'s

return a pointer into the internal buffer rather than copying into a
user-supplied one. this permits zero-copy in future use cases.

src/drv_imap.c

@@ -824,7 +824,8 @@ parse_imap_list( imap_store_t *ctx, char **sp, parse_list_state_t *sts )
 {
 	list_t *cur, **curp;
 	char *s = *sp, *d, *p;
-	int n, bytes;
+	int bytes;
+	uint n;
 	char c;
 
 	assert( sts );
@@ -882,12 +883,14 @@ parse_imap_list( imap_store_t *ctx, char **sp, parse_list_state_t *sts )
 			s[cur->len] = 0;
 
 		  getbytes:
-			n = socket_read( &ctx->conn, s, (uint)bytes );
-			if (n < 0) {
+			if (!(p = socket_read( &ctx->conn, 1, (uint)bytes, &n )))
+				goto postpone;
+			if (p == (void *)~0) {
 			  badeof:
 				sts->err = "unexpected EOF";
 				goto bail;
 			}
+			memcpy( s, p, n );
 			bytes -= n;
 			if (bytes > 0)
 				goto postpone;

src/socket.c

@@ -799,20 +799,30 @@ socket_expect_eof( conn_t *sock )
 #endif
 }
 
-int
-socket_read( conn_t *conn, char *buf, uint len )
+char *
+socket_read( conn_t *conn, uint min_len, uint max_len, uint *out_len )
 {
-	uint n = conn->bytes;
-	if (!n && conn->state == SCK_EOF)
-		return -1;
-	if (n > len)
-		n = len;
-	memcpy( buf, conn->buf + conn->offset, n );
-	if (!(conn->bytes -= n))
-		conn->offset = 0;
-	else
-		conn->offset += n;
-	return (int)n;
+	assert( min_len > 0 );
+	assert( min_len <= sizeof(conn->buf) );
+	assert( min_len <= max_len );
+
+	uint off = conn->offset;
+	uint cnt = conn->bytes;
+	if (cnt < min_len) {
+		if (conn->state == SCK_EOF)
+			return (void *)~0;
+		if (off + min_len > sizeof(conn->buf)) {
+			memmove( conn->buf, conn->buf + off, cnt );
+			conn->offset = 0;
+		}
+		return NULL;
+	}
+	uint n = (cnt < max_len) ? cnt : max_len;
+	cnt -= n;
+	conn->offset = cnt ? off + n : 0;
+	conn->bytes = cnt;
+	*out_len = n;
+	return conn->buf + off;
 }
 
 char *

src/socket.h

@@ -128,8 +128,9 @@ void socket_start_deflate( conn_t *conn );
 void socket_close( conn_t *sock );
 void socket_expect_activity( conn_t *sock, int expect );
 void socket_expect_eof( conn_t *sock );
-int socket_read( conn_t *sock, char *buf, uint len ); /* never waits */
-char *socket_read_line( conn_t *sock ); /* don't free return value; never waits */
+// Don't free return values. These functions never wait.
+char *socket_read( conn_t *conn, uint min_len, uint max_len, uint *out_len );
+char *socket_read_line( conn_t *conn );
 typedef enum { KeepOwn = 0, GiveOwn } ownership_t;
 typedef struct {
 	char *buf;