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Debugging

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Zhen Zhang edited this page Apr 27, 2018 · 14 revisions

General tips on debugging Haskell

https://wiki.haskell.org/Debugging

General tips on debugging shake

http://shakebuild.com/debugging

Make-based old system

  1. make show! VALUE=$(NAME)$ to check the value of variable NAME

Debugging Hadrian

General thoughts

  1. If a build command with some flags doesn't work, will that work on the Make-based old system? (Note that the directory structure is different, so we need to do some conversion)
  2. If you delete something, but run top-level target again doesn't produce that, then it must be that this things is not correctly registered as tracked output

Verbosity

Adding -V, -VV, -VVV can output more information from Shake and Hadrian for debugging purpose. For example, to print out the complete commands, you can use -V.

Common pitfalls

  • Out-of-date UserSetting.hs? (may cause compilation errors while building hadrian itself)
  • Path: absolute? relative? platform-dependent?
  • Missing environment variables?
  • The upstream GHC/Cabal/... has updated

How to read exception message

Type 1: openFile: does not exist

Example: hadrian: _build/stage1/ghc/GHCi/UI.o: openFile: does not exist (No such file or directory)

Cause: The build system tries to execute some raw I/O operation, which is not captured properly by shake build or any other handling mechanism.

Solution: Use shake-provided or Util-provided I/O actions which can automatically make input files dependencies, and throw better exception (like below) when no rule for building these files exists.

Type 2: Error when running Shake build system:

Example:

Error when running Shake build system:
* OracleQ (PackageDataKey ("_build/stage1/libraries/unix/package-data.mk","COMPONENT_ID"))
* _build/stage1/libraries/unix/package-data.mk
* OracleQ (PackageDataKey ("_build/stage1/libraries/base/package-data.mk","COMPONENT_ID"))
* _build/stage1/libraries/base/package-data.mk
* OracleQ (PackageDataKey ("_build/stage1/libraries/ghc-prim/package-data.mk","COMPONENT_ID"))
* _build/stage1/libraries/ghc-prim/package-data.mk
* _build/stage1/libraries/ghc-prim/GHC/Prim.hs
* inplace/bin/genprimopcode
* OracleQ (PackageDataKey ("_build/stage0/utils/genprimopcode/package-data.mk","C_SRCS"))
* _build/stage0/utils/genprimopcode/package-data.mk
user error (Development.Shake.cmd, system command failed
Command: _build/stage0/utils/ghc-cabal/ghc-cabal configure utils/genprimopcode /Users/zz/Repos/ghc2/_build/stage0/utils/genprimopcode '' --with-ghc=/usr/local/bin/ghc --with-ghc-pkg=/usr/local/bin/ghc-pkg --package-db=_build/stage0/bootstrapping.conf --enable-library-vanilla --enable-library-for-ghci --disable-library-profiling --disable-shared --with-hscolour=/Users/zz/Library/Haskell/bin/HsColour '--configure-option=CFLAGS=-fno-stack-protector -I/Users/zz/Repos/ghc2/_build/generated' --configure-option=CPPFLAGS=-I_build/generated '--gcc-options=-fno-stack-protector -I/Users/zz/Repos/ghc2/_build/generated' --configure-option=--with-cc=/usr/bin/gcc --constraint 'Cabal == 2.0.0.0' --constraint 'binary == 0.8.4.1' --constraint 'ghc-boot == 8.3' --constraint 'ghc-boot-th == 8.3' --constraint 'ghci == 8.3' --constraint 'hoopl == 3.10.2.2' --constraint 'hpc == 0.6.0.3' --constraint 'template-haskell == 2.12.0.0' --constraint 'terminfo == 0.4.1.0' --constraint 'transformers == 0.5.2.0' --with-gcc=/usr/bin/gcc --with-ar=/usr/bin/ar --with-alex=/Users/zz/Library/Haskell/bin/alex --with-happy=/Users/zz/Library/Haskell/bin/happy -v0 --configure-option=--quiet --configure-option=--disable-option-checking
Exit code: 1
Stderr:
ghc-cabal: ghc-pkg dump failed: dieVerbatim: user error (ghc-cabal:
'/usr/local/bin/ghc-pkg' exited with an error:
ghc-pkg: _build/stage0/bootstrapping.conf: getDirectoryContents: does not
exist (No such file or directory)
)
)

First, the list of OracleQs is similar to a call trace. Each OracleQ line specifies the target, and lines without OracleQ after it specify dependencies. Each dependency is the target of the next level.

The last level's target in the above example is _build/stage0/utils/genprimopcode/package-data.mk. Build system tries to build it with the command as shown in Command line (this is very useful -- since you can copy & paste and tweak with it directly when trying find the right rule).

The error dumped after the Command line can help you identify the potential bug quickly.

Run some snippets

Sometimes it is useful to run a few lines of code for debugging purpose, for example, print out the dependencies of cabal. The fastest way to do this is to modify the Main.hs, comment out the heavy-lifting rules, add insert the code you'd like to run.

        Rules.Clean.cleanRules
        Rules.Oracles.oracleRules
        -- Rules.SourceDist.sourceDistRules
        -- Rules.Selftest.selftestRules
        -- Rules.Test.testRules
        -- Rules.buildRules
        -- Rules.topLevelTargets
        rulesToTest

How to enable stack trace

first, build shake with profile support:

# inside shake source directory
# cabal sandbox init # you may want this
cabal install --enable-profiling

# for an example source file
cabal exec -- ghc -prof -fprof-auto examples/Main.hs

and a work-in-progress traced fork of shake

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