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<h3>
Ben Gamari pushed to branch wip/T18069
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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Commits:
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<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/1b7719bfa0c19f68e357cd0de7a742dd2d92d44c">1b7719bf</a></strong>
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<span>by Ben Gamari</span>
<i>at 2020-05-20T12:25:31-04:00</i>
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<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">SysTools.Process: Handle exceptions in readCreateProcessWithExitCode'
In #18069 we are observing MVar deadlocks from somewhere in ghc.exe.
This use of MVar stood out as being one of the more likely culprits.
Here we make sure that it is exception-safe.
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<h4>1 changed file:</h4>
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compiler/GHC/SysTools/Process.hs
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<h4>Changes:</h4>
<li id="7d3359a337486f9067d22455a38fdf75479921fb">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/1b7719bfa0c19f68e357cd0de7a742dd2d92d44c#7d3359a337486f9067d22455a38fdf75479921fb"><strong>compiler/GHC/SysTools/Process.hs</strong></a>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -41,6 +41,15 @@ enableProcessJobs opts = opts { use_process_jobs = True }</td>
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<pre style="margin: 0;"> <span id="LC41" class="line" lang="haskell"><span class="n" style="color: #333;">enableProcessJobs</span> <span class="n" style="color: #333;">opts</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">opts</span></span>
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<pre style="margin: 0;"> <span id="LC42" class="line" lang="haskell"><span class="o" style="font-weight: 600;">#</span><span class="n" style="color: #333;">endif</span></span>
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<pre style="margin: 0;"> <span id="LC43" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC44" class="line" lang="haskell"><span class="o" style="font-weight: 600;">#</span><span class="kr" style="font-weight: 600;">if</span> <span class="o" style="font-weight: 600;">!</span><span class="kt" style="color: #458; font-weight: 600;">MIN_VERSION_base</span><span class="p">(</span><span class="mi" style="color: #099;">4</span><span class="p">,</span><span class="mi" style="color: #099;">15</span><span class="p">,</span><span class="mi" style="color: #099;">0</span><span class="p">)</span></span>
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<pre style="margin: 0;">+<span id="LC45" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- TODO: This can be dropped with GHC 8.16</span></span>
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<pre style="margin: 0;">+<span id="LC46" class="line" lang="haskell"><span class="n" style="color: #333;">hGetContents'</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">Handle</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">IO</span> <span class="kt" style="color: #458; font-weight: 600;">String</span></span>
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<pre style="margin: 0;">+<span id="LC47" class="line" lang="haskell"><span class="n" style="color: #333;">hGetContents'</span> <span class="n" style="color: #333;">hdl</span> <span class="o" style="font-weight: 600;">=</span> <span class="kr" style="font-weight: 600;">do</span></span>
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<pre style="margin: 0;">+<span id="LC48" class="line" lang="haskell"> <span class="n" style="color: #333;">output</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">hGetContents</span> <span class="n" style="color: #333;">hdl</span></span>
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<pre style="margin: 0;">+<span id="LC49" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">_</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">evaluate</span> <span class="o" style="font-weight: 600;">$</span> <span class="n" style="color: #333;">length</span> <span class="n" style="color: #333;">output</span></span>
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<pre style="margin: 0;">+<span id="LC50" class="line" lang="haskell"> <span class="n" style="color: #333;">return</span> <span class="n" style="color: #333;">output</span></span>
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<pre style="margin: 0;">+<span id="LC51" class="line" lang="haskell"><span class="o" style="font-weight: 600;">#</span><span class="n" style="color: #333;">endif</span></span>
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<pre style="margin: 0;">+<span id="LC52" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC53" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- Similar to System.Process.readCreateProcessWithExitCode, but stderr is</span></span>
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<pre style="margin: 0;"> <span id="LC54" class="line" lang="haskell"><span class="c1" style="color: #998; font-style: italic;">-- inherited from the parent process, and output to stderr is not captured.</span></span>
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<pre style="margin: 0;"> <span id="LC55" class="line" lang="haskell"><span class="n" style="color: #333;">readCreateProcessWithExitCode'</span></span>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -51,13 +60,19 @@ readCreateProcessWithExitCode' proc = do</td>
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<pre style="margin: 0;"> <span id="LC60" class="line" lang="haskell"> <span class="n" style="color: #333;">createProcess</span> <span class="n" style="color: #333;">proc</span><span class="p">{</span> <span class="n" style="color: #333;">std_out</span> <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">CreatePipe</span> <span class="p">}</span></span>
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<pre style="margin: 0;"> <span id="LC61" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC62" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- fork off a thread to start consuming the output</span></span>
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<pre style="margin: 0;">-<span id="LC54" class="line" lang="haskell"> <span class="n" style="color: #333;">output</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">hGetContents</span> <span class="n" style="color: #333;">outh</span></span>
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<pre style="margin: 0;"> <span id="LC63" class="line" lang="haskell"> <span class="n" style="color: #333;">outMVar</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">newEmptyMVar</span></span>
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56
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<pre style="margin: 0;">-<span id="LC56" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">_</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">forkIO</span> <span class="o" style="font-weight: 600;">$</span> <span class="n" style="color: #333;">evaluate</span> <span class="p">(</span><span class="n" style="color: #333;">length</span> <span class="n" style="color: #333;">output</span><span class="p">)</span> <span class="o" style="font-weight: 600;">>></span> <span class="n" style="color: #333;">putMVar</span> <span class="n" style="color: #333;">outMVar</span> <span class="nb" style="color: #0086b3;">()</span></span>
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64
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<pre style="margin: 0;">+<span id="LC64" class="line" lang="haskell"> <span class="kr" style="font-weight: 600;">let</span> <span class="n" style="color: #333;">onError</span> <span class="o" style="font-weight: 600;">::</span> <span class="kt" style="color: #458; font-weight: 600;">SomeException</span> <span class="o" style="font-weight: 600;">-></span> <span class="kt" style="color: #458; font-weight: 600;">IO</span> <span class="nb" style="color: #0086b3;">()</span></span>
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65
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<pre style="margin: 0;">+<span id="LC65" class="line" lang="haskell"> <span class="n" style="color: #333;">onError</span> <span class="n" style="color: #333;">exc</span> <span class="o" style="font-weight: 600;">=</span> <span class="n" style="color: #333;">putMVar</span> <span class="n" style="color: #333;">outMVar</span> <span class="p">(</span><span class="kt" style="color: #458; font-weight: 600;">Left</span> <span class="n" style="color: #333;">exc</span><span class="p">)</span></span>
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66
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<pre style="margin: 0;">+<span id="LC66" class="line" lang="haskell"> <span class="n" style="color: #333;">forkIO</span> <span class="o" style="font-weight: 600;">$</span> <span class="n" style="color: #333;">handle</span> <span class="n" style="color: #333;">onError</span> <span class="o" style="font-weight: 600;">$</span> <span class="kr" style="font-weight: 600;">do</span></span>
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67
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<pre style="margin: 0;">+<span id="LC67" class="line" lang="haskell"> <span class="n" style="color: #333;">output</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">hGetContents'</span> <span class="n" style="color: #333;">outh</span></span>
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68
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<pre style="margin: 0;">+<span id="LC68" class="line" lang="haskell"> <span class="n" style="color: #333;">putMVar</span> <span class="n" style="color: #333;">outMVar</span> <span class="o" style="font-weight: 600;">$</span> <span class="kt" style="color: #458; font-weight: 600;">Right</span> <span class="n" style="color: #333;">output</span></span>
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57
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69
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<pre style="margin: 0;"> <span id="LC69" class="line" lang="haskell"></span>
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58
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<pre style="margin: 0;"> <span id="LC70" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- wait on the output</span></span>
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59
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<pre style="margin: 0;">-<span id="LC59" class="line" lang="haskell"> <span class="n" style="color: #333;">takeMVar</span> <span class="n" style="color: #333;">outMVar</span></span>
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71
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<pre style="margin: 0;">+<span id="LC71" class="line" lang="haskell"> <span class="n" style="color: #333;"><span class="idiff left">result</span></span><span class="idiff"> </span><span class="o" style="font-weight: 600;"><span class="idiff"><-</span></span><span class="idiff right"> </span><span class="n" style="color: #333;">takeMVar</span> <span class="n" style="color: #333;">outMVar</span></span>
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60
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72
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<pre style="margin: 0;"> <span id="LC72" class="line" lang="haskell"> <span class="n" style="color: #333;">hClose</span> <span class="n" style="color: #333;">outh</span></span>
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73
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<pre style="margin: 0;">+<span id="LC73" class="line" lang="haskell"> <span class="n" style="color: #333;">output</span> <span class="o" style="font-weight: 600;"><-</span> <span class="kr" style="font-weight: 600;">case</span> <span class="n" style="color: #333;">result</span> <span class="kr" style="font-weight: 600;">of</span></span>
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74
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<pre style="margin: 0;">+<span id="LC74" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">Left</span> <span class="n" style="color: #333;">exc</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">throwIO</span> <span class="n" style="color: #333;">exc</span></span>
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75
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<pre style="margin: 0;">+<span id="LC75" class="line" lang="haskell"> <span class="kt" style="color: #458; font-weight: 600;">Right</span> <span class="n" style="color: #333;">output</span> <span class="o" style="font-weight: 600;">-></span> <span class="n" style="color: #333;">return</span> <span class="n" style="color: #333;">output</span></span>
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61
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76
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<pre style="margin: 0;"> <span id="LC76" class="line" lang="haskell"></span>
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62
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77
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<pre style="margin: 0;"> <span id="LC77" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- wait on the process</span></span>
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63
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78
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<pre style="margin: 0;"> <span id="LC78" class="line" lang="haskell"> <span class="n" style="color: #333;">ex</span> <span class="o" style="font-weight: 600;"><-</span> <span class="n" style="color: #333;">waitForProcess</span> <span class="n" style="color: #333;">pid</span></span>
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