This comment contains my original proposal.

An updated version can be found below (in #33848 (comment)).

# Upgrade the golang.org/x/sys module and fetch it from $GOPROXY.
# Also ensure that the rest of the module dependencies are consistent
# with the requirements of the upgraded module.
# This step updates the go.mod file, but not the vendor directory.
$ go get -d golang.org/x/sys

# A plain 'go test ./…' at this point may fail,
# because vendor/modules.txt may be out of sync with the go.mod file.
# However, 'go test -mod=mod ./…' should succeed,
# because it explicitly ignores the vendor directory.
$ go test -mod=mod ./...

# Download any newly-upgraded dependencies to the module cache,
# and copy them from the module cache to the vendor directory.
$ go mod vendor

# Test the packages within the module
# using the newly-updated dependencies in the vendor directory.
$ go test ./...

One point I'm a bit uncertain on: above I proposed to trigger the automatic -mod=vendor behavior based on the presence of a ## annotation in the vendor/modules.txt file.

Would it be better to instead trigger it based on a go 1.14 directive in the go.mod file (plus the existence of vendor/modules.txt? (Why or why not?)

That seems like it would be simpler to understand, including because I think it has been previously stated that the format of the modules.txt file is an internal and undocumented implementation detail?

Could you even take it a step further to have it trigger off of the presence of the vendor directory itself plus go 1.14 in the go.mod, rather than vendor/modules.txt plus go 1.14?

It would then be triggering off of things that are both more visible and actually documented, and hence would likely be easier to understand.

I propose to enable -mod=vendor automatically, and to fail build operations when -mod=vendor is enabled if the vendor/ directory is out of sync with the go.mod file.

When I build in a container, I will be copying the entire projects source tree of code into that container manually. This will cause things to be out of sync. So now, I will have to make sure that I don't copy the go.mod|sum files into the container.

This also means that anyone else on my team working with this project, will need to run throught the proxy server to validate their vendor folder is "correct". I am concerned that in the end, the vendor folder is not really acting as a vendor folder. If the module system doesn't like what it sees in vendor, the build breaks. The deps needs to be cached anyway. So might as well not even use vendor.

When I build in a container, I will be copying the entire projects source tree of code into that container manually. This will cause things to be out of sync. So now, I will have to make sure that I don't copy the go.mod|sum files into the container.

I think I'm missing something. How would copying the entire source tree into the container cause the vendor directory to get out of sync with the go.mod file? Presumably you'd be copying in the same go.mod file that you used to generate the vendor directory in the first place.

How are you going to verify that the vendor folder and the go.mod file is in sync? This is going to require network calls against the proxy server I assume? I am wondering if the deps are going to be downloaded as well during this process?

@ardan-bkennedy, that's all covered in the text of the proposal above (#33848 (comment)).

I read that several more times and I think it’s starting to make sense. What the above proposal is saying is, given a vendor folder with a modules.txt file, just verify that this file is in sync with the go.mod file? If they are, build against the vendor folder. If they are not, issue a build error? Therefore no calls to the proxy server are required. The original workflow is maintained?

Yep. Being able to do that without hitting the network at all requires that we include some extra information in the vendor/modules.txt file compared to what we're writing today, but the amount of extra information seems reasonable to me, and the resulting file should be backward-compatible to earlier Go releases.

That's brilliant and allows vendoring to be an option again. Which is much appreciated. So the next question is, will this make a point release for 1.13 or do we need to wait for 1.14?

My concern is that projects may just give up on vendoring because of the current breakage in workflows. Which is a shame because I think vendoring has strong advantages when it's reasonable to use it.

This would not be backported to 1.13. (Per the minor release policy, we only backport “security issues, serious problems with no workaround, and documentation fixes”.)

In the interim, users who rely on vendoring workflows can enable vendoring for all build commands by setting GOFLAGS=-mod=vendor, explicitly disable module mode in Go 1.13 by setting GO111MODULE=off, or implicitly disable module mode within GOPATH/src by staying on Go 1.12 until they're ready to try a 1.14 pre-release.

It sounds like the general consensus here is that we should accept this proposal for Go 1.14.
This is probably important to do before we set GO111MODULE=on for people using the dev branch.

Am I reading this wrong? Does anyone object to accepting this proposal? Thanks.

An interesting wrinkle: if a replaced module does not contribute any imported packages, we omit it from vendor/modules.txt. However, that replacement can still have an effect on the overall build list, because the replacement go.mod file may differ from the original.

This implies that we need to duplicate (and verify) all replace entries found in the go.mod file to modules.txt, even if they do not affect any module in the actual build list.

If we're not loading transitive dependencies, we don't know whether a replaced module actually exists in the build list, so we can't add an entry of the form # <module> <version>: otherwise, older versions of the go command will erroneously interpret them as being in the build list.

Fortunately, the old modules.txt parser skips module entries that have additional text after the version, so the following styles of lines should be ignored (and thus safe to use).

# example.org/unused v1.0.0 (unused)
## replaced by github.com/unused v1.0.0
# example.com/other/unused * (unused)
## replaced by ./other-unused

Well that's funny. The current go mod vendor implementation already writes replacements, using the form # MODULE VERSION => MODULE [VERSION]:

I think that means that go mod vendor is currently writing vendor/modules.txt files that it isn`t capable of parsing correctly.

Change https://golang.org/cl/198319 mentions this issue: cmd/go: automatically check and use vendored packages

I have updated the proposal below, based on information learned during prototyping. This version supersedes the one in #33848 (comment).

Proposal

I propose to enable -mod=vendor automatically for operations that do not explicitly query or manipulate the build list, and to fail build operations when -mod=vendor is enabled if the vendor/ directory is out of sync with the go.mod file.

In order to detect when the vendor/ directory is out of sync with the go.mod file, we need to know the constraints imposed by the main module — that is, its require and replace directives. Given those, the remaining module requirements are computed deterministically, so we do not need to store or check the complete module graph, which may be much larger than the explicit requirements or even the final list of selected versions.

Concretely, the changes are:

• In the vendor/modules.txt file, add an annotation indicating which modules were listed explicitly in a require or replace directive in the go.mod file as of the last invocation of go mod vendor. (This implies that every module path and version that appears in the main module's go.mod file must be listed in modules.txt, even if no packages are vendored from within that module or version.)

  • To maintain compatibility with the Go 1.11 modules.txt parser, this annotation must have the following properties:

    • Does not change the number of fields in the # <module-path> <version> lines [edit: for unreplaced modules only], which must be equal to 3 in order for the existing parser to recognize the line.
    • Does not occur as a line with exactly one field (which would be interpreted as a package path within the preceding module).
    • Either does not have the prefix # , or does not occur in between the # <module-path> <version> lines and the associated <package-path> lines. (Otherwise, it would break the association between modules and packages.)

  • [edit: Adopted the existing syntax generated by go mod vendor for replacements, as discovered above .]
    To comply with the above requirements, I propose that we use a line of the form:

    # example.com/some/module vA.B.C
    ## explicit
    

    to annotate explicit dependencies, and

    # example.com/some/module vA.B.C => example.com/some/other/module vX.Y.Z
    

    for replacements.

  • To allow for future expansion, the updated parser should check lines beginning with ## for any semicolon-delimited field matching the expected annotation, ignoring any whitespace surrounding the field. (All other tokens within such comments are reserved for future use.)

• [edit: Adopted @thepudds' suggestion for trigger condition.]
  When the go.mod file specifies go 1.14 and a top-level vendor directory exists, then for all build commands except go get:

  • Change the default value of the -mod build flag to vendor.

  • Verify that the set of modules annotated as explicit in vendor/modules.txt is exactly equal to the set of modules (and versions) found in require directives in the main module's go.mod file.

  • [edit: Expanded this point to ensure that the replacements in the go.mod file account for all of the replacements reported in vendor/modules.txt.]
    Verify that the set of replacements indicated in vendor/modules.txt includes the complete set of replace directives (including both path and version, if applicable) in the main module's go.mod file, and that the replacements in the go.mod file account for all of the module replacements reported in vendor/modules.txt.

• Add a new value for the -mod flag, -mod=mod, to explicitly request the non-vendor behavior: that is, to ignore the contents of the vendor directory and to automatically update the go.mod file as needed.

  • The go get command today treats an explicit -mod=vendor flag as an error. With this change, go get should instead always provide the behavior of -mod=mod, treating the -mod flag as unrecognized (see cmd/go: readonly does not affect all go mod sub-commands #26850 (comment)).

    In particular, go get should ignore any implicit -mod=vendor from the presence of a vendor/modules.txt file or from a -mod flag set via GOFLAGS (see also cmd/go: go get can't fetch tools with mod=vendor #32502), and treat an explicit -mod=vendor, -mod=readonly, or -mod=mod as an error (see cmd/go: explicit '-mod=readonly' argument to go get is not rejected #30345).

  • [edit: Added discussion of go list.]
    go list -m command today accepts -mod=vendor, and displays a truncated version of the build list (restricted to what was found in the vendor/modules.txt file) without further warning. Implicitly truncating the build list seems like a mistake.

    Since -mod=vendor will now be enabled implicitly, that difference will become even more subtle, especially since go mod graph will not similarly truncate the graph. I propose that go list -m should instead fail with a helpful error if -mod=vendor is set.

The proposed approach allows the following commands to work from a vendor directory with an empty module cache and no network access:

• go list -deps ./...
• go build ./...
• go test ./…

Explicitly build-list related commands will continue to download missing modules to the cache:

• go mod graph
• go list -m example.com/some/module

Other commands may fail without an explicit -mod=mod override:

• go list all
• go test all
• go list -test $(go list -deps ./…)

Note that the go mod subcommands do not have a -mod= flag and therefore never look at the vendor directory for information. They always require access to a module cache or module proxy. The check in this proposal ensures that the results produced by any go mod subcommand remain accurate even when the corresponding build commands use vendored sources.

Example workflow

# Upgrade the golang.org/x/sys module and fetch it from $GOPROXY.
# Also ensure that the rest of the module dependencies are consistent
# with the requirements of the upgraded module.
# This step updates the go.mod file, but not the vendor directory.
$ go get -d golang.org/x/sys

# A plain 'go test ./…' at this point may fail,
# because vendor/modules.txt may be out of sync with the go.mod file.
# However, 'go test -mod=mod ./…' should succeed,
# because it explicitly ignores the vendor directory.
$ go test -mod=mod ./...

# Download any newly-upgraded dependencies to the module cache,
# and copy them from the module cache to the vendor directory.
$ go mod vendor

# Test the packages within the module
# using the newly-updated dependencies in the vendor directory.
$ go test ./...

Caveats

Intentional modifications

The above checks do not detect intentional modifications to the vendored sources. Users who want to ensure that no intentional modifications are present will need to re-run go mod vendor and look for diffs, or (after #27348 is addressed) run go mod verify.

Local filesystem changes

The above checks also do not detect skew due to changes in local files within a directory specified in a replace directive. I do not recommend the use of filesystem replace directives in general, but the potential for skew makes them particularly important to avoid when using a vendor directory.

If we believe that changes in the local filesystem will be a significant source of skew in practice, we could expand the replaced by annotation to include a hash of the replacement directory. However, computing that hash could significantly increase the cost of the consistency check, which in the current proposal only requires us to read the go.mod and vendor/modules.txt files.

Dependency analysis

Because the approach proposed here does not retain go.mod files for modules that do not contribute packages to the build (including older-than-selected versions of packages that do contribute packages), the vendor directory will not support analysis of the module graph (go list -m all, go mod why -m, and go mod graph).

That seems like an acceptable tradeoff: adding those go.mod files would require some scheme for encoding their versions within the vendor directory, which we do not have today, and would introduce diffs that are largely irrelevant to the build during code review.

Change https://golang.org/cl/198318 mentions this issue: internal/goversion: update to 1.14

The (unused) annotations turn out to be very complex to implement, and for relatively little gain in usability. Probably we should leave them out.

Updated #33848 (comment) and the prototype CL to omit the (unused) annotations.

The prototype implementation (CL 198319 and CL 198438) is ready for review.

Change https://golang.org/cl/207397 mentions this issue: cmd/go: document the -mod=mod option

Change https://golang.org/cl/210341 mentions this issue: cmd/go: include cfg.BuildModReason in 'import lookup disabled' errors

Change https://golang.org/cl/214081 mentions this issue: cmd/go: explicitly reject 'list -u' and 'list -versions' when '-mod=vendor' is set

Change https://golang.org/cl/222538 mentions this issue: vendor: migrate from govendor to go mod vendor

Change https://golang.org/cl/251159 mentions this issue: cmd/go/internal/modload: remove (*loader).forceStdVendor