Avoid crashing with a corrupted lockfile

I did a bad thing (script that edits the Gemfile.lock directly) and
ended up with a Gemfile.lock that was completely missing some indirect
dependencies. While this is my fault and an error is reasonable, I
noticed that the error got progressively less friendly in recent
versions of bundler.

Something similar came up in #6210,
and this commit would have helped with that case as well
(although we've already handled this a different way with #6219).

Details:
---

Back on Bundler 2.2.23, a corrupt lockfile like this would cause a helpful error:

```
Unable to find a spec satisfying minitest (>= 5.1) in the set. Perhaps the lockfile is corrupted?
```

Bundler 2.3.26 gave a helpful warning:

```
Warning:
Your lockfile was created by an old Bundler that left some things out.
Because of the missing DEPENDENCIES, we can only install gems one at a time,
instead of installing 16 at a time.
You can fix this by adding the missing gems to your Gemfile, running bundle
install, and then removing the gems from your Gemfile.
The missing gems are:
* minitest depended upon by activesupport
```

But then continued on and crashed while trying to report the unmet
dependency:

```
--- ERROR REPORT TEMPLATE -------------------------------------------------------

NoMethodError: undefined method `full_name' for nil:NilClass
lib/bundler/installer/parallel_installer.rb:127:in `block (2 levels) in check_for_unmet_dependencies'

...
```

Bundler 2.4.0 and up crash as above when jobs=1, but crash
even harder when run in parallel:

```
--- ERROR REPORT TEMPLATE -------------------------------------------------------

fatal: No live threads left. Deadlock?
3 threads, 3 sleeps current:0x00007fa6b6704660 main thread:0x00007fa6b6704660
* #<Thread:0x000000010833b130 sleep_forever>
   rb_thread_t:0x00007fa6b6704660 native:0x0000000108985600 int:0

* #<Thread:0x0000000108dea630@Parallel Installer Worker #0 tmp/1/gems/system/gems/bundler-2.5.0.dev/lib/bundler/worker.rb:90 sleep_forever>
   rb_thread_t:0x00007fa6b67f67c0 native:0x0000700009a62000 int:0

* #<Thread:0x0000000108dea4a0@Parallel Installer Worker #1 tmp/1/gems/system/gems/bundler-2.5.0.dev/lib/bundler/worker.rb:90 sleep_forever>
   rb_thread_t:0x00007fa6b67f63c0 native:0x0000700009c65000 int:0

<internal:thread_sync>:18:in `pop'
tmp/1/gems/system/gems/bundler-2.5.0.dev/lib/bundler/worker.rb:42:in `deq'

...
```

Changes
---

This commit fixes the confusing thread deadlock crash by detecting if
dependencies are missing such that we'll never be able to enqueue. When
that happens we treat it as a failure so the install can finish.

That gets us back to the `NoMethodError`, which this commit fixes by
using a different warning in the case where no spec is found.

bundler/lib/bundler/installer/parallel_installer.rb

@@ -47,6 +47,13 @@ def dependencies_installed?(all_specs)
         dependencies.all? {|d| installed_specs.include? d.name }
       end
 
+      # Check whether spec's dependencies are missing, which can indicate a
+      # corrupted lockfile
+      def dependencies_missing?(all_specs)
+        spec_names = all_specs.map(&:name)
+        dependencies.any? {|d| !spec_names.include? d.name }
+      end
+
       # Represents only the non-development dependencies, the ones that are
       # itself and are in the total list.
       def dependencies
@@ -110,12 +117,17 @@ def check_for_unmet_dependencies
 
       warning = []
       warning << "Your lockfile doesn't include a valid resolution."
-      warning << "You can fix this by regenerating your lockfile or trying to manually editing the bad locked gems to a version that satisfies all dependencies."
+      warning << "You can fix this by regenerating your lockfile or manually editing the bad locked gems to a version that satisfies all dependencies."
       warning << "The unmet dependencies are:"
 
       unmet_dependencies.each do |spec, unmet_spec_dependencies|
         unmet_spec_dependencies.each do |unmet_spec_dependency|
-          warning << "* #{unmet_spec_dependency}, depended upon #{spec.full_name}, unsatisfied by #{@specs.find {|s| s.name == unmet_spec_dependency.name && !unmet_spec_dependency.matches_spec?(s.spec) }.full_name}"
+          found = @specs.find {|s| s.name == unmet_spec_dependency.name && !unmet_spec_dependency.matches_spec?(s.spec) }
+          if found
+            warning << "* #{unmet_spec_dependency}, depended upon by #{spec.full_name}, unsatisfied by #{found.full_name}"
+          else
+            warning << "* #{unmet_spec_dependency}, depended upon by #{spec.full_name} but missing from lockfile"
+          end
         end
       end
 
@@ -212,6 +224,8 @@ def enqueue_specs
         if spec.dependencies_installed? @specs
           spec.state = :enqueued
           worker_pool.enq spec
+        elsif spec.dependencies_missing? @specs
+          spec.state = :failed
         end
       end
     end

bundler/spec/lock/lockfile_spec.rb

@@ -1221,6 +1221,44 @@
       and include("Either installing with `--full-index` or running `bundle update rack_middleware` should fix the problem.")
   end
 
+  it "errors gracefully on a corrupt lockfile" do
+    build_repo4 do
+      build_gem "minitest-bisect", "1.6.0" do |s|
+        s.add_dependency "path_expander", "~> 1.1"
+      end
+
+      build_gem "path_expander", "1.1.1"
+    end
+
+    gemfile <<~G
+      source "#{file_uri_for(gem_repo4)}"
+      gem "minitest-bisect"
+    G
+
+    # Corrupt lockfile (completely missing path_expander)
+    lockfile <<~L
+      GEM
+        remote: #{file_uri_for(gem_repo4)}/
+        specs:
+          minitest-bisect (1.6.0)
+
+      PLATFORMS
+        #{lockfile_platforms}
+
+      DEPENDENCIES
+        minitest-bisect
+
+      BUNDLED WITH
+         #{Bundler::VERSION}
+    L
+
+    cache_gems "minitest-bisect-1.6.0", "path_expander-1.1.1", :gem_repo => gem_repo4
+    bundle :install, :raise_on_error => false
+
+    expect(err).not_to include("ERROR REPORT TEMPLATE")
+    expect(err).to include("path_expander (~> 1.1), depended upon by minitest-bisect-1.6.0 but missing from lockfile")
+  end
+
   it "auto-heals when the lockfile is missing specs" do
     build_repo4 do
       build_gem "minitest-bisect", "1.6.0" do |s|

bundler/spec/support/helpers.rb

@@ -414,14 +414,14 @@ def realworld_system_gems(*gems)
       end
     end
 
-    def cache_gems(*gems)
+    def cache_gems(*gems, gem_repo: gem_repo1)
       gems = gems.flatten
 
       FileUtils.rm_rf("#{bundled_app}/vendor/cache")
       FileUtils.mkdir_p("#{bundled_app}/vendor/cache")
 
       gems.each do |g|
-        path = "#{gem_repo1}/gems/#{g}.gem"
+        path = "#{gem_repo}/gems/#{g}.gem"
         raise "OMG `#{path}` does not exist!" unless File.exist?(path)
         FileUtils.cp(path, "#{bundled_app}/vendor/cache")
       end