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Fix PNG loader not resetting stream position on error right after reading signature #387

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Merged
merged 3 commits into from
Jan 25, 2025
Merged

Fix PNG loader not resetting stream position on error right after reading signature #387

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Core (LV::Video): Fix PNG loader not resetting stream position on err…
kaixiong Jan 24, 2025
2cfeb2f
Core (LV::Video): Fix and clarify the various seeks in the PNG loader.
kaixiong Jan 24, 2025
2ef768f
Core (LV::Video): Use 'delete[] ptr' instead of 'delete []ptr'.
kaixiong Jan 24, 2025
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7 changes: 7 additions & 0 deletions libvisual/libvisual/private/lv_video_png.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,11 @@ namespace LV {

png_byte signature[8];
input.read (reinterpret_cast<char*> (signature), sizeof (signature));
input.seekg (saved_stream_pos);
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@hartwork hartwork Jan 24, 2025

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  • Please add a comment to this line why we're rewinding the string here, the code does not make it obvious.
  • Is there a chance that seekg could kill an error state flag set by read above? I'm having trouble finding docs whether error flags are set until reset by the user or being auto-reset by every new API call?
  • PS: Does this have any effect on the input.seekg (saved_stream_pos); in line 93 being still needed or no longer needed?

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@kaixiong kaixiong Jan 24, 2025 •
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  • Please add a comment to this line why we're rewinding the string here, the code does not make it obvious.

  • Is there a chance that seekg could kill an error state flag set by read above? I'm having trouble finding docs whether error flags are set until reset by the user or being auto-reset by every new API call?

According to this, seekg():

  1. Clears the eof flag before anything else
  2. Sets the input position only if fail() != true (i.e. either bad or fail flags are set)

So there are two failure scenarios:

  1. Partial read of signature. Both eof and fail flags are set, so seekg() does nothing. Clearly bad.
  2. Complete read of signature but stream ends immediately (no image data), so only eof is set. seekg() clears the eof and resets the input position. Okay, the decoding runs after and read errors will be caught there and then.
  • PS: Does this have any effect on the input.seekg (saved_stream_pos); in line 93 being still needed or no longer needed?

Did you mean line 98? Indeed, any read failure prior to that clean up routine will prevent the position reset, so we need to clear the fail flag with clear().

So, back to the drawing board..


if (!input) {
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@hartwork hartwork Jan 24, 2025

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(This reads rather weirdly to me, could we use .bad instead maybe? (https://cplusplus.com/reference/ios/ios/bad/))

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@kaixiong kaixiong Jan 24, 2025

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The ! operator tests for the 'fail' and 'bad' flags, so they're not equivalent. However, neither accounts for EOF it seems (I thought ! did). It looks like we ought to use !input.good() 😄

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@hartwork hartwork Jan 24, 2025

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@kaixiong I would like !good a lot better 👍

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@kaixiong kaixiong Jan 24, 2025

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@hartwork, sorry bad() is indeed equivalent to !. I seriously hate iostream.

return nullptr;
}

bool is_png = !png_sig_cmp (signature, 0, sizeof (signature));

Expand Down Expand Up @@ -100,6 +105,8 @@ namespace LV {
return nullptr;
}

input.seekg (sizeof (signature), std::ios::cur);

png_set_read_fn (png_ptr, &input, handle_png_read);

png_set_sig_bytes (png_ptr, sizeof (signature));
Expand Down