[FFmpeg-trac] #8550(avformat:closed): Cannot open DShow capture device twice with same settings

[FFmpeg]

#8550: Cannot open DShow capture device twice with same settings
-------------------------------------+-------------------------------------
             Reporter:  uzyfeo       |                    Owner:
                 Type:  defect       |                   Status:  closed
             Priority:  normal       |                Component:  avformat
              Version:  unspecified  |               Resolution:  invalid
             Keywords:               |               Blocked By:
  dshow,win32,mfc                    |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Changes (by uzyfeo):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 We tried to compile ffmpeg ourselves from 4.2.2 sources and use it as
 static library instead of shared. We compiled it using VS2019 C++ build
 tools with debug symbols enabled. After we stepped through code, we found
 out cause of the issue, although as of now not its origin.

 During first run, when avformat_open_input is called, AVFormatCtx it
 initializes has during its passage through dshow_cycle_formats (found in
 ffmpeg-4.2.2/libavdevice/dshow.c) value of framerate 0.

 After reinitializing capture, it has framerate set to 30.00050 or
 something like that. This casues chack in function dshow_cycle_formats at
 line

 Found out the culprit. Although, further investigation is still pending.
 It seems that, for whatever reason, when specifying framerate that should
 be used, this value is remembered and passed down until program is closed.

 Value is preserved in AVFormatCtx under name framerate. I dont know
 particular structure as of right now or exact details of how it got set
 and why it did not got reset properly, but will surely have a look in a
 bit.

 I found the problem after stepping through half of the code, investigating
 each failed function call. Value in this structure, when it works as
 intended, is 0. When it fails, it fails because this value was set to
 30.0035 or something like that, which is just a tad bit above max limit it
 checks against.

 We never set this value explicitly ourselves in AVFormatContext returned
 by avformat_open_input, so this value must have been set somewhere after
 that single check and didn't got cleared properly later on or we are
 accidentally setting something up we should. As said, no direct
 modifications were done on these values.

 Function and check where it fails and it should have not is in
 ffmpeg-4.2.2/libavdevice/dshow.c at 396.
 As ctx->framerate is 0 during first run, this check is not performed.
 After first run, this value is set when it most probably shouldn't, and if
 it should, that check should take into account floating point errors
 properly.

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Ticket URL: <https://trac.ffmpeg.org/ticket/8550#comment:1>
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