dep: Catch DeltaWriter diff bookkeeping errors

Relates to #1945.

While this doesn't fix the issue, it at least prevents a panic from
occurring that blows up a `dep ensure` run, and should give us enough
information to debug it if/when someone runs into it.

docs/glossary.md

@@ -108,7 +108,7 @@ The root import path for a project. A project root is defined as:
 * For the current project, the location of the `Gopkg.toml` file defines the project root
 * For dependencies, the root of the network [source](#source) (VCS repository) is treated as the project root
 
-These are generally one and the same, though not always. When using dep inside a monorepo, multiple `Gopkg.toml` files may exist at subpaths for discrete projects, designating each of those import paths as Project Roots. This works fine when working directly on those projects. If, however, any project not in the repository seeks to import the monorepo, dep will treat the monorepo's as one big Project, with the root directory being the Project Root; it will disregard any and all `Gopkg.toml` files in subdirectories.
+These are generally one and the same, though not always. When using dep inside a monorepo, multiple `Gopkg.toml` files may exist at subpaths for discrete projects, designating each of those import paths as Project Roots. This works fine when working directly on those projects. If, however, any project not in the repository seeks to import the monorepo, dep will treat the monorepo as one big Project, with the root directory being the Project Root; it will disregard any and all `Gopkg.toml` files in subdirectories.
 
 This may also be referred to as the "import root" or "root import path."
 

txn_writer.go

@@ -536,7 +536,15 @@ func (dw *DeltaWriter) Write(path string, sm gps.SourceManager, examples bool, l
 		}
 
 		to := filepath.FromSlash(filepath.Join(vnewpath, string(pr)))
-		po := projs[pr].(verify.VerifiableProject).PruneOpts
+		po, has := projs[pr].(verify.VerifiableProject).PruneOpts
+		if !has {
+			// This shouldn't be reachable, but it's preferable to print an
+			// error and continue rather than panic. https://github.com/golang/dep/issues/1945
+			// TODO(sdboyer) remove this once we've increased confidence around
+			// this case.
+			fmt.Fprintf(os.Stderr, "Internal error - %s had change code %v but was not in new Gopkg.lock. Re-running dep ensure should fix this. Please file a bug!\n", pr, reason)
+			continue
+		}
 		if err := sm.ExportPrunedProject(context.TODO(), projs[pr], po, to); err != nil {
 			return errors.Wrapf(err, "failed to export %s", pr)
 		}

website/i18n/en.json

@@ -7,6 +7,7 @@
     "daily-dep": "Daily Dep",
     "deduction": "Import Path Deduction",
     "ensure-mechanics": "Models and Mechanisms",
+    "env-vars": "Environment Variables",
     "failure-modes": "Failure Modes",
     "FAQ": "FAQ",
     "glossary": "Glossary",