x: Dynamically configure INCR chunk size

src/clipserve.c

@@ -12,12 +12,12 @@
 #include "util.h"
 #include "x.h"
 
-#define INCR_CHUNK_BYTES 4096
-
 static struct incr_transfer *it_list = NULL;
 static Display *dpy;
 static Atom incr_atom;
 
+static size_t chunk_size;
+
 /**
  * Start an INCR transfer.
  */
@@ -37,7 +37,6 @@ static void incr_send_start(XSelectionRequestEvent *req,
         .data = (char *)content->data,
         .data_size = content->size,
         .offset = 0,
-        .chunk_size = INCR_CHUNK_BYTES,
     };
 
     it_dbg(it, "Starting transfer\n");
@@ -70,19 +69,19 @@ static void incr_send_chunk(const XPropertyEvent *pe) {
     while (it) {
         if (it->requestor == pe->window && it->property == pe->atom) {
             size_t remaining = it->data_size - it->offset;
-            size_t chunk_size =
-                (remaining > it->chunk_size) ? it->chunk_size : remaining;
+            size_t this_chunk_size =
+                (remaining > chunk_size) ? chunk_size : remaining;
 
             it_dbg(it,
                    "Sending chunk (bytes sent: %zu, bytes remaining: %zu)\n",
                    it->offset, remaining);
 
-            if (chunk_size > 0) {
+            if (this_chunk_size > 0) {
                 XChangeProperty(dpy, it->requestor, it->property, it->target,
                                 it->format, PropModeReplace,
                                 (unsigned char *)(it->data + it->offset),
-                                chunk_size);
-                it->offset += chunk_size;
+                                this_chunk_size);
+                it->offset += this_chunk_size;
             } else {
                 incr_send_finish(it);
             }
@@ -107,6 +106,8 @@ static void _nonnull_ serve_clipboard(uint64_t hash,
     dpy = XOpenDisplay(NULL);
     expect(dpy);
 
+    chunk_size = get_chunk_size(dpy);
+
     win = XCreateSimpleWindow(dpy, DefaultRootWindow(dpy), 0, 0, 1, 1, 0, 0, 0);
     XStoreName(dpy, win, "clipserve");
     targets = XInternAtom(dpy, "TARGETS", False);
@@ -147,7 +148,7 @@ static void _nonnull_ serve_clipboard(uint64_t hash,
                                     arrlen(available_targets));
                 } else if (req->target == utf8_string ||
                            req->target == XA_STRING) {
-                    if (content->size <= INCR_CHUNK_BYTES) {
+                    if (content->size < (off_t)chunk_size) {
                         // Data size is small enough, send directly
                         XChangeProperty(dpy, req->requestor, req->property,
                                         req->target, 8, PropModeReplace,

src/x.c

@@ -53,6 +53,24 @@ int xerror_handler(Display *dpy _unused_, XErrorEvent *ee) {
         ee->error_code);
 }
 
+#define FALLBACK_CHUNK_BYTES 4 * 1024
+
+/**
+ * Calculate and cache an appropriate INCR chunk size.
+ *
+ * We consider selections larger than a quarter of the maximum request size to
+ * be "large". That's what others (like xclip) do, so it's clearly ok in
+ * practice.
+ */
+size_t get_chunk_size(Display *dpy) {
+    // Units are 4-byte words, so this is 1/4 in bytes
+    size_t chunk_size = XExtendedMaxRequestSize(dpy);
+    if (chunk_size == 0) {
+        chunk_size = XMaxRequestSize(dpy);
+    }
+    return chunk_size ? chunk_size / 4 : FALLBACK_CHUNK_BYTES;
+}
+
 /**
  * Add a new INCR transfer to the active list.
  */

src/x.h

@@ -7,6 +7,7 @@
 
 DEFINE_DROP_FUNC_VOID(XFree)
 
+size_t _nonnull_ get_chunk_size(Display *dpy);
 char _nonnull_ *get_window_title(Display *dpy, Window owner);
 int xerror_handler(Display *dpy _unused_, XErrorEvent *ee);
 
@@ -21,7 +22,6 @@ struct incr_transfer {
     size_t data_size;
     size_t data_capacity;
     size_t offset;
-    size_t chunk_size;
 };
 
 #define it_dbg(it, fmt, ...)                                                   \