GUAC-236: Implement PNG decoding using Cairo's built-in PNG functions.

src/guacenc/image-stream.c

@@ -133,8 +133,11 @@ int guacenc_image_stream_end(guacenc_image_stream* stream,
         return 1;
 
     /* Draw surface to buffer */
+    if (buffer->cairo != NULL) {
         cairo_set_source_surface(buffer->cairo, surface, stream->x, stream->y);
         cairo_fill(buffer->cairo);
+    }
+
     cairo_surface_destroy(surface);
     return 0;
 

src/guacenc/png.c

@@ -21,14 +21,92 @@
  */
 
 #include "config.h"
+#include "log.h"
 #include "png.h"
 
 #include <cairo/cairo.h>
 
 #include <stdlib.h>
+#include <string.h>
+
+/**
+ * The current state of the PNG decoder.
+ */
+typedef struct guacenc_png_read_state {
+
+    /**
+     * The buffer of unread image data. This pointer will be updated to point
+     * to the next unread byte when data is read.
+     */
+    unsigned char* data;
+
+    /**
+     * The number of bytes remaining to be read within the buffer.
+     */
+    unsigned int length;
+
+} guacenc_png_read_state;
+
+/**
+ * Attempts to fill the given buffer with read image data. The behavior of
+ * this function is dictated by cairo_read_t.
+ *
+ * @param closure
+ *     The current state of the PNG decoding process (an instance of
+ *     guacenc_png_read_state).
+ *
+ * @param data
+ *     The data buffer to fill.
+ *
+ * @param length
+ *     The number of bytes to fill within the data buffer.
+ *
+ * @return
+ *     CAIRO_STATUS_SUCCESS if all data was read successfully (the entire
+ *     buffer was filled), CAIRO_STATUS_READ_ERROR otherwise.
+ */
+static cairo_status_t guacenc_png_read(void* closure, unsigned char* data,
+        unsigned int length) {
+
+    guacenc_png_read_state* state = (guacenc_png_read_state*) closure;
+
+    /* If more data is requested than is available in buffer, fail */
+    if (length > state->length)
+        return CAIRO_STATUS_READ_ERROR;
+
+    /* Read chunk into buffer */
+    memcpy(data, state->data, length);
+
+    /* Advance to next chunk */
+    state->length -= length;
+    state->data += length;
+
+    /* Read was successful */
+    return CAIRO_STATUS_SUCCESS;
+
+}
 
 cairo_surface_t* guacenc_png_decoder(unsigned char* data, int length) {
-    /* STUB */
+
+    guacenc_png_read_state state = {
+        .data = data,
+        .length = length
+    };
+
+    /* Read PNG from data */
+    cairo_surface_t* surface =
+        cairo_image_surface_create_from_png_stream(guacenc_png_read, &state);
+
+    /* If surface returned with an error, just return NULL */
+    if (surface != NULL &&
+            cairo_surface_status(surface) != CAIRO_STATUS_SUCCESS) {
+        guacenc_log(GUAC_LOG_WARNING, "Invalid PNG data");
+        cairo_surface_destroy(surface);
         return NULL;
     }
 
+    /* PNG was read successfully */
+    return surface;
+
+}
+