Document an overview of builds

With all of the various refactorings at play, I wanted to document what
the end goal of this whole process would be, both to guide my own work,
an to get feedback from others (either to improve the process, or
clarify it so others can participate in the maintenance of this better).

doc/build-overview.md

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+<div class="hidden-warning"><a href="https://docs.haskellstack.org/"><img src="https://rawgit.com/commercialhaskell/stack/master/doc/img/hidden-warning.svg"></a></div>
+
+# Build Overview
+
+__NOTE__ This document should *not be considered accurate* until this
+note is removed.
+
+This is a work-in-progress document covering the build process used by Stack.
+Stack. It was started following the Pantry rewrite work in Stack (likely to
+land as Stack 2.0), and contains some significant changes/simplifications from
+how things used to work. This document will likely not fully be reflected in
+the behavior of Stack itself until late in the Stack 2.0 development cycle.
+
+## Terminology
+
+* Project package: anything listed in `packages` in stack.yaml
+* Dependency: anything listed in extra-deps or a snapshot
+* Target: package and/or component listed on the command line to be built. Can
+  be either project package or dependency. If none specified, automatically
+  targets all project packages
+* Immutable package: a package which comes from Hackage, an archive, or a
+  repository. In contrast to...
+* Mutable package: a package which comes from a local file path. The contents
+  of such a package are assumed to mutate over time.
+* Snapshot database: a package database and set of executables for a given set
+  of _immutable_ packages. Only packages from immutable sources and which
+  depend exclusively on other immutable packages can be in this database.
+  *QUESTION* Would this better be called the _immutable database_ or _write only
+  database_?
+* Local database: a package database and set of executables for packages which
+  are either mutable or depend on such mutable packages. Importantly, packages
+  in this database can be unregister, replaced, etc, depending on what happens
+  with the source packages.
+
+Outdated terminology to be purged:
+
+* Wanted
+* Local
+* Snapshot package
+
+## Inputs
+
+Stack pays attention to the following inputs:
+
+* Current working directory, used for finding the default `stack.yaml` file and
+  resolving relative paths
+* The `STACK_YAML` environment variable
+* Command line arguments (CLI args), as will be referenced below
+
+Given these inputs, Stack attempts the following process when performing a build.
+
+## Find the `stack.yaml` file
+
+* Check for a `--stack-yaml` CLI arg, and use that
+* Check for a `STACK_YAML` env var
+* Look for a `stack.yaml` in this directory or ancestor directories
+* Fall back to the default global project
+
+This file is parsed to provide the following config values:
+
+* `resolver`
+* `compiler`
+* `packages`
+* `extra-deps`
+* `flags`
+* `ghc-options`
+
+`flags` and `ghc-options` break down into both _by name_ (applied to a
+specific package) and _general_.
+
+## Wanted compiler, dependencies, and project packages
+
+* If the `--resolver` CLI is present, ignore the `resolver` and
+  `compiler` config values
+* Load up the snapshot indicated by the `resolver` (either config
+  value or CLI arg). This will provide:
+    * A map from package name to package location, flags, GHC options,
+      and if a package should be hidden. All package locations here
+      are immutable.
+    * A wanted compiler version, e.g. `ghc-8.4.3`
+* If the `--compiler` CLI arg is set, or the `compiler` config value
+  is set (and `--resolver` CLI arg is not set), ignore the wanted
+  compiler from the snapshot and use the specified wanted compiler
+* Parse `extra-deps` into a `Map PackageName PackageLocation`,
+  containing both mutable and immutable package locations. Parse
+  `packages` into a `Map PackageName ProjectPackage`.
+* Ensure there are no duplicates between these two sets of packages
+* Delete any packages from the snapshot packages that appear in
+  `packages` or `extra-deps`
+* Perform a left biased union between the immutable `extra-deps`
+  values and the snapshot packages. Ignore any settings in the
+  snapshot packages that have been replaced.
+* Apply the `flags` and `ghc-options` by name to these packages. If
+  any values are specified but no matching package is found, it's an
+  error.
+* We are now left with the following:
+    * A wanted compiler version
+    * A map from package name to immutable packages with package config (flags, GHC options, hidden)
+    * A map from package name to mutable packages as dependencies with package config
+    * A map from package name to mutable packages as project packages with package config
+
+## Get actual compiler
+
+Use the wanted compiler and various other Stack config values (not all
+listed here) to find the actual compiler, potentially installing it in
+the process.
+
+## Global package sources
+
+With the actual compiler discovered, list out the packages available
+in its database and create a map from package name to
+version/GhcPkgId. Remove any packages from this map which are present
+in one of the other three maps mentioned above.
+
+## Resolve targets
+
+Take the CLI args for targets as raw text values and turn them into
+actual targets.
+
+* Do a basic parse of the values into one of the following:
+    * Package name
+    * Package identifier
+    * Package name + component
+    * Directory
+* An empty target list is equivalent to listing the package names of
+  all project packages
+* For any directories specified, find all project packages in that
+  directory or subdirectories therefore and convert to those package
+  names
+* For all package identifiers, ensure that either the package name
+  does not exist in any of the three parsed maps from the "wanted
+  compiler" step above, or that the package is present as an immutable
+  dependency from Hackage. If so, create an immutable dependency entry
+  with default flags, GHC options, and hidden status, and add this
+  package to the set of immutable package dependencies.
+* For all package names, ensure the package is in one of the four maps
+  we have, and if so add to either the dependency or project package
+  target set.
+* For all package name + component, ensure that the package is a
+  project package, and add that package + component to the set of
+  project targets.
+* Ensure that no target has been specified multiple times.
+
+We now have an update four package maps, a new set of dependency
+targets, and a new set of project package targets (potentially with
+specific components).
+
+## Apply named CLI flags
+
+Named CLI flags are applied to specific packages by updating the
+config in one of the four maps. If a flag is specified and no package
+is found, it's an error.
+
+## Apply CLI GHC options
+
+Apply GHC options from the command line to all _project package
+targets_. *FIXME* confirm that this is in fact the correct behavior.
+
+## Apply general flags (CLI and config value)
+
+*FIXME* figure out and document exactly which packages these will
+apply to.
+
+## Apply general GHC options
+
+*FIXME* list out the various choices here and which packages they
+apply to.
+
+## Determine snapshot hash
+
+Use some deterministic binary serialization and SHA256 thereof to get
+a hash of the following information:
+
+* Actual compiler (GHC version, path, *FIXME* probably some other
+  unique info from GHC, I've heard that `ghc --info` gives you
+  something)
+* Global database map
+* Immutable dependency map
+
+Motivation: Any package built from the immutable dependency map and
+installed in this database will never need to be rebuilt.
+
+*FIXME* Caveat: do we need to take profiling settings into account
+here? How about Haddock status?
+
+## Determine actual target components
+
+* Dependencies: "default" components (all libraries and executables)
+* Project packages:
+    * If specific components named: only those, plus any libraries present
+    * If no specific components, include the following:
+        * All libraries, always
+        * All executables, always
+        * All test suites, _if_ `--test` specified on command line
+        * All benchmarks, _if_ `--bench` specified on command line
+
+## Construct build plan
+
+* Applied to every target (project package or dependency)
+* Apply flags, platform, and actual GHC version to resolve
+  dependencies in any package analyzed
+* Include all library dependencies for all enabled components
+* Include all build tool dependencies for all enabled components
+  (using the fun backwards compat logic for `build-tools`)
+* Apply the logic recursively to come up with a full build plan
+* If a task depends exclusively on immutable packages, mark it as
+  immutable. Otherwise, it's mutable. The former go into the snapshot
+  database, the latter into the local database.
+
+We now have a set of tasks of packages/components to build, with full
+config information for each package, and dependencies that must be
+built first.
+
+*FIXME* There's some logic to deal with cyclic dependencies between
+test suites and benchmarks, where a task can be broken up into
+individual components versus be kept as a single task. Need to
+document this better. Currently it's the "all in one" logic.
+
+## Unregister local modified packages
+
+* For all mutable packages in the set of tasks, see if any files have
+  changed since last successful build and, if so, unregister + delete
+  their executables
+* For anything which depends on them directly or transitively,
+  unregister + delete their executables
+
+## Perform the tasks
+
+* Topological sort, find things which have no dependencies remaining
+* Check if already installed in the relevant database
+    * Check package database
+    * Check Stack specific "is installed" flags, necessary for
+      non-library packages
+    * For project packages, need to also check which components were
+      built, if tests were run, if we need to rerun tests, etc
+* If all good: do nothing
+* Otherwise, for immutable tasks: check the precompiled cache for an
+  identical package installation (same GHC, dependencies, etc). If
+  present: copy that over, and we're done.
+* Otherwise, perform the build, register, write to the Stack specific
+  "is installed" stuff, and (for immutable tasks) register to the
+  precompiled cache
+
+"Perform the build" consists of:
+
+* Do a cabal configure, if needed
+* Build the desired components
+* For all test suites built, unless "no rerun tests" logic is on and
+  we already ran the test, _or_ "no run tests" is on, run the test
+* For all benchmarks built, unless "no run benchmarks" is on, run the
+  benchmark