Optimization for the performance of addModules(), addExplodedModules(), and addFlattenModules()

[edithwuly]

Summary

Reduce the number of times to traverse buildpack bits. Save about 27% time spent on addExplodedModules().
Call NToLayerTar in goroutine. Save more than half time spent on addFlattenModules().

Description

Before

Each buildpack bits will be traversed three times: download, calculate diffID and stream to daemon.
NToLayerTar in addFlattenModules() is called for each flattenModule sequentially.

After

The diffID of buildpack will calculated while it is downloaded and saved as a tarfile.
NToLayerTar is called in goroutine, referring to addExplodedModules().

Profiling Result

command	average time spent on addExplodedModules() before	average time spent on addExplodedModules() after	ratio
pack build demo-app --builder cnbs/sample-builder:bionic -p ../samples/java/maven --buildpack paketobuildpacks/java:latest	3.804439914s	2.746395149s	0.72189115

command	average time spent on addFlattenModules() before	average time spent on addFlattenModules() after	ratio
pack builder create cnbs/sample-builder:alpine --config builder.toml --flatten	7.761502674s	3.751928196s	0.48340229

# builder.toml
# Buildpacks to include in builder
[[buildpacks]]
uri = "../samples/buildpacks/hello-processes"

[[buildpacks]]
# Packaged buildpacks to include in builder;
# the "hello-universe" package contains the "hello-world" and "hello-moon" buildpacks
uri = "docker://paketobuildpacks/java:latest"

# Order used for detection
[[order]]
    # This buildpack will create a process type "sys-info" to display runtime information
    [[order.group]]
    id = "samples/hello-processes"
    version = "0.0.1"

# Stack that will be used by the builder
[stack]
id = "io.buildpacks.samples.stacks.bionic"
# This image is used at runtime
run-image = "cnbs/sample-stack-run:bionic"
# This image is used at build-time
build-image = "cnbs/sample-stack-build:bionic"

Documentation

• Should this change be documented?
  • Yes, see #___
  • No

Related

Resolves #___

[natalieparellano]

Awesome work @edithwuly! Just a note to maintainers, we might want to hold this PR until we can update the benchmark suite here to show the impact of this change by using a larger buildpack (the samples one is only a few KB it turns out).

[natalieparellano]

As per usual it seems Codecov is being a bit too picky here... I wonder if there is a way we can get it to ignore error checks, at least on i/o operations 🤔

[natalieparellano]

@buildpacks/platform-maintainers could you please merge this PR first: #1773

[jjbustamante]

Great job @edithwuly! Thanks for parallelize the logic for the flatten feature!! ❤️

[natalieparellano]

Summarizing offline discussion - I think we're planning to fold this PR into some work that @jjbustamante is doing (as there will likely be a merge conflict)

[jjbustamante]

I am closing this PR based on the latest conversation regarding this feature and the after the RFC is approved, we will remove most the current implementation and this is not going to be valid anymore