[pango] Add assertions to guard against division by zero on sample text paths

[Philip Withnall <pwithnall src gnome org>]

commit e4ea2d2e5118b44bf1c3e5fa998356696e073aa8
Author: Philip Withnall <withnall endlessm com>
Date:   Tue Feb 14 11:12:51 2017 +0000

    Add assertions to guard against division by zero on sample text paths
    
    There are a few code paths where pango_utf8_strwidth() is called on
    language-specific sample text. The sample text should have been chosen
    to never have a zero width, but we should add some assertions to ensure
    that’s the case. This guides static analysers into the right analysis.
    
    Coverity IDs: 1391697, 1391698, 1391699
    
    Signed-off-by: Philip Withnall <withnall endlessm com>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=778602

 pango/pango-context.c   |    8 +++++++-
 pango/pangocairo-font.c |    5 ++++-
 pango/pangofc-font.c    |    5 ++++-
 3 files changed, 15 insertions(+), 3 deletions(-)
---
diff --git a/pango/pango-context.c b/pango/pango-context.c
index f91d0fe..f0cea73 100644
--- a/pango/pango-context.c
+++ b/pango/pango-context.c
@@ -1635,6 +1635,10 @@ update_metrics_from_items (PangoFontMetrics *metrics,
   GHashTable *fonts_seen = g_hash_table_new (NULL, NULL);
   PangoGlyphString *glyphs = pango_glyph_string_new ();
   GList *l;
+  glong text_width;
+
+  /* This should typically be called with a sample text string. */
+  g_return_if_fail (text_len > 0);
 
   metrics->approximate_char_width = 0;
 
@@ -1663,7 +1667,9 @@ update_metrics_from_items (PangoFontMetrics *metrics,
   pango_glyph_string_free (glyphs);
   g_hash_table_destroy (fonts_seen);
 
-  metrics->approximate_char_width /= pango_utf8_strwidth (text);
+  text_width = pango_utf8_strwidth (text);
+  g_assert (text_width > 0);
+  metrics->approximate_char_width /= text_width;
 }
 
 /**
diff --git a/pango/pangocairo-font.c b/pango/pangocairo-font.c
index 97df00c..b86f85b 100644
--- a/pango/pangocairo-font.c
+++ b/pango/pangocairo-font.c
@@ -249,6 +249,7 @@ _pango_cairo_font_get_metrics (PangoFont     *font,
       cairo_matrix_t cairo_matrix;
       PangoMatrix pango_matrix;
       PangoMatrix identity = PANGO_MATRIX_INIT;
+      glong sample_str_width;
 
       int height, shift;
 
@@ -311,7 +312,9 @@ _pango_cairo_font_get_metrics (PangoFont     *font,
       pango_layout_set_text (layout, sample_str, -1);
       pango_layout_get_extents (layout, NULL, &extents);
 
-      info->metrics->approximate_char_width = extents.width / pango_utf8_strwidth (sample_str);
+      sample_str_width = pango_utf8_strwidth (sample_str);
+      g_assert (sample_str_width > 0);
+      info->metrics->approximate_char_width = extents.width / sample_str_width;
 
       pango_layout_set_text (layout, "0123456789", -1);
       info->metrics->approximate_digit_width = max_glyph_width (layout);
diff --git a/pango/pangofc-font.c b/pango/pangofc-font.c
index b463e90..c35b77e 100644
--- a/pango/pangofc-font.c
+++ b/pango/pangofc-font.c
@@ -588,6 +588,7 @@ pango_fc_font_get_metrics (PangoFont     *font,
        PangoRectangle extents;
        const char *sample_str = pango_language_get_sample_string (language);
        PangoFontDescription *desc = pango_font_describe_with_absolute_size (font);
+       gulong sample_str_width;
 
         layout = pango_layout_new (context);
        pango_layout_set_font_description (layout, desc);
@@ -596,7 +597,9 @@ pango_fc_font_get_metrics (PangoFont     *font,
        pango_layout_set_text (layout, sample_str, -1);
        pango_layout_get_extents (layout, NULL, &extents);
 
-       info->metrics->approximate_char_width = extents.width / pango_utf8_strwidth (sample_str);
+       sample_str_width = pango_utf8_strwidth (sample_str);
+       g_assert (sample_str_width > 0);
+       info->metrics->approximate_char_width = extents.width / sample_str_width;
 
        pango_layout_set_text (layout, "0123456789", -1);
        info->metrics->approximate_digit_width = max_glyph_width (layout);