Get window image using SHM, if possible

If MIT-SHM extension is available, maintain a SHM segment appropriately sized to
hold the window image (i.e. with the same lifetime as the composite pixmap)

Try to get window image by copying it to a SHM image, before falling back to
copying it to a normal image (for which the image data must be read over the
socket from the server)

6 files changed, 113 insertions, 15 deletions

diff --git a/configure.ac b/configure.ac
index b848f5b..e20af3d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -20,7 +20,7 @@ AC_PROG_OBJC
 AC_PROG_INSTALL
 
 # Checks for libraries.
-NEEDED="xcb-damage xcb-composite xcb-event xcb-xtest xcb-image xcb-keysyms xcb-icccm >= 0.3.9 xcb-atom xcb-ewmh"
+NEEDED="xcb-damage xcb-composite xcb-event xcb-xtest xcb-image xcb-keysyms xcb-icccm >= 0.3.9 xcb-atom xcb-ewmh xcb-shm"
 PKG_CHECK_MODULES(XCB, $NEEDED)
 AC_SUBST(NEEDED)
 
diff --git a/src/libxcwm/context.c b/src/libxcwm/context.c
index e092fab..57b3460 100644
--- a/src/libxcwm/context.c
+++ b/src/libxcwm/context.c
@@ -119,6 +119,9 @@ xcwm_context_open(char *display)
     if (!_xcwm_init_extension(conn, "XKEYBOARD"))
         goto fail;
 
+    context->has_shm = (_xcwm_init_extension(conn, "MIT-SHM") != NULL);
+    context->depth = xcb_aux_get_depth(conn, root_screen);
+
     _xcwm_atoms_init(context);
 
     /* Select for XFIXES cursor notify events */
diff --git a/src/libxcwm/event_loop.c b/src/libxcwm/event_loop.c
index 702d6ae..b6769fe 100644
--- a/src/libxcwm/event_loop.c
+++ b/src/libxcwm/event_loop.c
@@ -28,6 +28,8 @@
 #endif
 
 #include <pthread.h>
+#include <sys/shm.h>
+
 #include <xcwm/xcwm.h>
 #include <xcb/composite.h>
 #include "xcwm_internal.h"
@@ -105,19 +107,65 @@ _xcwm_event_stop_loop(void)
 static void
 _xcwm_window_composite_pixmap_release(xcwm_window_t *window)
 {
-  if (window->composite_pixmap_id)
-    {
+    /* Release composite pixmap */
+    if (window->composite_pixmap_id) {
       xcb_free_pixmap(window->context->conn, window->composite_pixmap_id);
       window->composite_pixmap_id = 0;
     }
+
+    /* Release SHM resources */
+    if (window->context->has_shm) {
+        if (window->shminfo.shmseg) {
+            xcb_shm_detach(window->context->conn, window->shminfo.shmseg);
+            window->shminfo.shmseg = 0;
+        }
+
+        if (window->shminfo.shmaddr != (void *)-1) {
+            shmdt(window->shminfo.shmaddr);
+            window->shminfo.shmaddr = (void *)-1;
+        }
+
+        if (window->shminfo.shmid != -1) {
+            shmctl(window->shminfo.shmid, IPC_RMID, 0);
+            window->shminfo.shmid = -1;
+        }
+    }
 }
 
 static void
 _xcwm_window_composite_pixmap_update(xcwm_window_t *window)
 {
-  _xcwm_window_composite_pixmap_release(window);
-  window->composite_pixmap_id = xcb_generate_id(window->context->conn);
-  xcb_composite_name_window_pixmap(window->context->conn, window->window_id, window->composite_pixmap_id);
+    _xcwm_window_composite_pixmap_release(window);
+
+    /* Assign a pixmap XID to the composite pixmap */
+    window->composite_pixmap_id = xcb_generate_id(window->context->conn);
+    xcb_composite_name_window_pixmap(window->context->conn, window->window_id, window->composite_pixmap_id);
+
+    if (window->context->has_shm)
+    {
+        /* Pre-compute the size of image data needed for this window */
+        xcb_image_t *image = xcb_image_create_native(window->context->conn,
+                                                 window->bounds.width,
+                                                 window->bounds.height,
+                                                 XCB_IMAGE_FORMAT_Z_PIXMAP,
+                                                 window->context->depth,
+                                                 NULL, ~0, NULL);
+        size_t image_size = image->size;
+        xcb_image_destroy(image);
+        printf("window 0x%08x requires %d bytes of SHM\n", window->window_id, image_size);
+
+        /* Allocate SHM resources */
+        window->shminfo.shmid = shmget(IPC_PRIVATE, image_size, IPC_CREAT | 0777);
+        if (window->shminfo.shmid < 0)
+            fprintf(stderr, "shmget failed\n");
+
+        window->shminfo.shmaddr = shmat(window->shminfo.shmid, 0, 0);
+        if (window->shminfo.shmaddr < 0)
+            fprintf(stderr, "shmget failed\n");
+
+        window->shminfo.shmseg = xcb_generate_id(window->context->conn);
+        xcb_shm_attach(window->context->conn, window->shminfo.shmseg, window->shminfo.shmid, 0);
+    }
 }
 
 /*
diff --git a/src/libxcwm/image.c b/src/libxcwm/image.c
index cb5ca26..34069d4 100644
--- a/src/libxcwm/image.c
+++ b/src/libxcwm/image.c
@@ -36,7 +36,7 @@
 xcwm_image_t *
 xcwm_image_copy_partial(xcwm_window_t *window, xcwm_rect_t *area)
 {
-    xcb_image_t *image;
+    xcb_image_t *image = NULL;
 
     /* Return null if image is 0 by 0 */
     if (area->width == 0 || area->height == 0) {
@@ -44,14 +44,55 @@ xcwm_image_copy_partial(xcwm_window_t *window, xcwm_rect_t *area)
     }
 
     /* Get the image of the specified area of the window */
-    image = xcb_image_get(window->context->conn,
-                          window->composite_pixmap_id,
-                          area->x,
-                          area->y,
-                          area->width,
-                          area->height,
-                          (unsigned int)~0L,
-                          XCB_IMAGE_FORMAT_Z_PIXMAP);
+
+    if (window->context->has_shm && window->shminfo.shmaddr) {
+        /*
+          A composite pixmap cannot be a shm pixmap, so we must use
+          xcb_image_shm_get to copy the image from the window's composite
+          pixmap to a shm image.
+
+          Rather than creating it every-time, we keep a shared memory
+          segment around which is of the right size for the window image.
+        */
+
+        /* Create an xcb_image_t for which we manage the data */
+        image = xcb_image_create_native(window->context->conn,
+                                        area->width,
+                                        area->height,
+                                        XCB_IMAGE_FORMAT_Z_PIXMAP,
+                                        window->context->depth,
+                                        NULL, ~0, NULL);
+        if (image) {
+            /* data is the shared memory */
+            image->data = window->shminfo.shmaddr;
+
+            /* Read image data from the composite pixmap into the shm image */
+            if (!xcb_image_shm_get(window->context->conn,
+                                   window->composite_pixmap_id,
+                                   image,
+                                   window->shminfo,
+                                   area->x,
+                                   area->y,
+                                   (unsigned int)~0L)) {
+                xcb_image_destroy(image);
+                image = NULL;
+            }
+        }
+    }
+
+    if (!image && window->context->has_shm)
+        printf("has_shm, but falling back to socket image\n");
+
+    /* If shm not available, or failed, use xcb_image_get() */
+    if (!image)
+        image = xcb_image_get(window->context->conn,
+                              window->composite_pixmap_id,
+                              area->x,
+                              area->y,
+                              area->width,
+                              area->height,
+                              (unsigned int)~0L,
+                              XCB_IMAGE_FORMAT_Z_PIXMAP);
 
     /* Failed to get a valid image, return null */
     if (!image) {
diff --git a/src/libxcwm/window.c b/src/libxcwm/window.c
index 0b227de..eafda92 100644
--- a/src/libxcwm/window.c
+++ b/src/libxcwm/window.c
@@ -120,6 +120,9 @@ _xcwm_window_create(xcwm_context_t *context, xcb_window_t new_window,
     window->notified_bounds = window->bounds;
     window->opacity = ~0;
     window->composite_pixmap_id = 0;
+    window->shminfo.shmseg = 0;
+    window->shminfo.shmaddr = (void *)-1;
+    window->shminfo.shmid = -1;
     window->local_data = 0;
     window->shape = 0;
 
diff --git a/src/libxcwm/xcwm_internal.h b/src/libxcwm/xcwm_internal.h
index 9e3b993..99c6e00 100644
--- a/src/libxcwm/xcwm_internal.h
+++ b/src/libxcwm/xcwm_internal.h
@@ -69,6 +69,8 @@ struct xcwm_context_t {
     int fixes_event_base;
     xcb_window_t wm_cm_window;
     xcwm_wm_atoms_t atoms;
+    int has_shm;
+    int depth;
 };
 
 /**
@@ -92,6 +94,7 @@ struct xcwm_window_t {
     void *local_data;   /* Area for data client cares about */
     unsigned int opacity;
     xcb_pixmap_t composite_pixmap_id;
+    xcb_shm_segment_info_t shminfo;
     xcb_shape_get_rectangles_reply_t *shape;
 };