preprocessor: install multiple versions of CUE in docker image

Currently we only install a single CUE version in the default site
docker image: v0.8.0-alpha.1. However, not all docs on the site require
installing a pre-release version.

Update the default image to install multiple versions under /cues.

We drive that installation by configuration. In the root of the site
configuration there is already a 'versions' field used to drive various
tool versions used in the base image and elsewhere. The 'cue' field of
the 'versions' field is now a struct, mapping a human name to a CUE
version:

    cue: {
        latest:     "v0.7.1"
        prerelease: "v0.8.0-alpha.1"
        tip:        "v0.8.0-alpha.1"
    }

This configuration is used to update the Dockerfile template.

As a bit of a hack (for now) we use the prerelease version of CUE as the
default in the default image. For guides that require an alternative
version, they can update PATH using a hidden script at the top of the
guide. In such cases it is good practice to then have a regular script
run to show 'cue version' so the reader is clear on the required version
for that guide.

We also update the PATH that is explicitly set in the generated bash
file. The TODO above that hack still holds; it is unclear why the
environment established in the Dockfile and entrypoint is not "good" at
the point of running our script.

The configuration map of CUE versions is not fully utilised in this CL.
Rather, at a later date, we will drive more of the site, labels etc
based on these versions, effectively migrating from the previous
(current) state of hanging everything off a single version. Those
changes will happen over time in subsequent CLs.

Another important usage of the configured CUE versions is in guides that
use the Go API. To make such guides reproducible, we 'go get' a specific
version of cuelang.org/go rather, that '@latest'. This requires us to
run a reader-visible command such as:

    go get cuelang.org/[email protected]

Rather than hard-code such a version, we want to use the CUE versions
configured at the site level, described above.

Currently we have no mechanism of referring from a script block to that
configuration.

This CL introduces a basic initial version that uses specially named
environment variables:

    {{{with script "en" "example"}}}
    go get cuelang.org/go@$CUELANG_CUE_PRERELEASE
    {{{end}}}

The environment variables follow a pattern CUELANG_CUE_$NAME where
$NAME refers to the human readable field name in the 'cue' struct
described above.

The substitution of these environment variables happens after the
parsing of script nodes, but prior to the running of the multi-step
script. This ensures that the actual command run includes the resolved
version. It also ensures that the rendered Hugo page includes the
resolved version, as opposed to an obscurely named environment variable.

The substitution of these environment variables happens via an adapted
version of os.Expand that conditionally substitutes environment
variables. This code is vendored alongside the preprocessor code with a
couple of tests to cover the extended functionality.

Preprocessor-No-Write-Cache: true
Signed-off-by: Paul Jolly <[email protected]>
Change-Id: Ie8f2da1cac137ce8c390b5c6d08dcb449c4ff78a

internal/cmd/preprocessor/cmd/_docker/Dockerfile

@@ -15,18 +15,37 @@ RUN \
   export GOCACHE=/cache/gocache GOMODCACHE=/cache/gomodcache && \
   go install -trimpath github.com/rogpeppe/go-internal/cmd/[email protected]
 
+RUN mkdir /cues
+
+RUN \
+  --mount=type=cache,target=/cache/gocache \
+  --mount=type=cache,target=/cache/gomodcache \
+  export GOCACHE=/cache/gocache GOMODCACHE=/cache/gomodcache && \
+  GOBIN=/cues/latest go install -trimpath cuelang.org/go/cmd/[email protected]
+
 RUN \
   --mount=type=cache,target=/cache/gocache \
   --mount=type=cache,target=/cache/gomodcache \
   export GOCACHE=/cache/gocache GOMODCACHE=/cache/gomodcache && \
-  go install -trimpath cuelang.org/go/cmd/[email protected]
+  GOBIN=/cues/prerelease go install -trimpath cuelang.org/go/cmd/[email protected]
+
+RUN \
+  --mount=type=cache,target=/cache/gocache \
+  --mount=type=cache,target=/cache/gomodcache \
+  export GOCACHE=/cache/gocache GOMODCACHE=/cache/gomodcache && \
+  GOBIN=/cues/tip go install -trimpath cuelang.org/go/cmd/[email protected]
 
 FROM golang:1.22.0
 
 RUN mkdir -p /go/bin
 
+# TODO: make this more principled with respect to cue.versions
+ENV PATH="/cues/prerelease:${PATH}"
+
 ENV PATH="/go/bin:/usr/local/go/bin:${PATH}"
-ENV CUE_VERSION="v0.8.0-alpha.1"
+ENV CUELANG_CUE_LATEST="v0.7.1"
+ENV CUELANG_CUE_PRERELEASE="v0.8.0-alpha.1"
+ENV CUELANG_CUE_TIP="v0.8.0-alpha.1"
 
 WORKDIR /
 
@@ -36,6 +55,8 @@ RUN chmod 755 /usr/bin/entrypoint.sh
 RUN chown root:root /usr/bin/entrypoint.sh
 
 COPY --from=build /go/bin/testscript /go/bin
-COPY --from=build /go/bin/cue /go/bin
+COPY --from=build /cues/latest/cue /cues/latest/cue
+COPY --from=build /cues/prerelease/cue /cues/prerelease/cue
+COPY --from=build /cues/tip/cue /cues/tip/cue
 
 ENTRYPOINT ["/usr/bin/entrypoint.sh"]

internal/cmd/preprocessor/cmd/execute.go

@@ -137,6 +137,23 @@ type executionContext struct {
 	// preprocessor.
 	config cue.Value
 
+	// cueVersions is a map of name to version of CUE versions configured on the
+	// site. e.g.
+	//
+	// latest => v0.7.1
+	// prerelease => v0.8.0-alpha.1
+	cueVersions map[string]string
+
+	// cueEnvVersions is a map of env var name to CUE versions, derived from
+	// cueVersions. For example the entry:
+	//
+	// latest => v0.7.0
+	//
+	// in cueVersions exists in this map as:
+	//
+	// CUELANG_CUE_LATEST => v0.7.0
+	cueEnvVersions map[string]string
+
 	// selfHash is the hash of the preprocessor itself that hashing calculations
 	// should use if they need the preprocessor to affect the hash result.
 	selfHash string

internal/cmd/preprocessor/cmd/execute_context.go

@@ -117,6 +117,21 @@ func (ec *executeContext) execute() error {
 	}
 	ec.config = v
 
+	// Load the CUE versions configured as part of the site
+	ec.cueVersions = make(map[string]string)
+	ec.cueEnvVersions = make(map[string]string)
+	cueVersionsPath := cue.ParsePath("versions.cue")
+	cueVersions := ec.config.LookupPath(cueVersionsPath)
+	if cueVersions.Exists() {
+		if err := cueVersions.Decode(&ec.cueVersions); err != nil {
+			return ec.errorf("%v: failed to decode %v: %v", cueVersionsPath, err)
+		}
+		for k, v := range ec.cueVersions {
+			key := fmt.Sprintf("CUELANG_CUE_%s", strings.ToUpper(k))
+			ec.cueEnvVersions[key] = v
+		}
+	}
+
 	// Now load config per page
 	for _, d := range ec.order {
 		p := ec.pages[d]

internal/cmd/preprocessor/cmd/expand.go

@@ -0,0 +1,104 @@
+// Copyright 2024 The CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cmd
+
+// The code below is copied and modified from the Go source as of
+// 9cc0f9cba2f2db97f5ba6c2c482bafaaa34c0381
+//
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// expand replaces ${var} or $var in the string based on the mapping function
+// if the mapping function returns true.
+func expand(s string, mapping func(string) (bool, string)) string {
+	var buf []byte
+	// ${} is all ASCII, so bytes are fine for this operation.
+	i := 0
+	for j := 0; j < len(s); j++ {
+		if s[j] == '$' && j+1 < len(s) {
+			if buf == nil {
+				buf = make([]byte, 0, 2*len(s))
+			}
+			buf = append(buf, s[i:j]...)
+			name, w := getShellName(s[j+1:])
+			if name == "" && w > 0 {
+				// Encountered invalid syntax; eat the
+				// characters.
+			} else if name == "" {
+				// Valid syntax, but $ was not followed by a
+				// name. Leave the dollar character untouched.
+				buf = append(buf, s[j])
+			} else {
+				mapped, repl := mapping(name)
+				if mapped {
+					buf = append(buf, repl...)
+				} else {
+					buf = append(buf, s[j:j+1+w]...)
+				}
+			}
+			j += w
+			i = j + 1
+		}
+	}
+	if buf == nil {
+		return s
+	}
+	return string(buf) + s[i:]
+}
+
+// isShellSpecialVar reports whether the character identifies a special
+// shell variable such as $*.
+func isShellSpecialVar(c uint8) bool {
+	switch c {
+	case '*', '#', '$', '@', '!', '?', '-', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
+		return true
+	}
+	return false
+}
+
+// isAlphaNum reports whether the byte is an ASCII letter, number, or underscore.
+func isAlphaNum(c uint8) bool {
+	return c == '_' || '0' <= c && c <= '9' || 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z'
+}
+
+// getShellName returns the name that begins the string and the number of bytes
+// consumed to extract it. If the name is enclosed in {}, it's part of a ${}
+// expansion and two more bytes are needed than the length of the name.
+func getShellName(s string) (string, int) {
+	switch {
+	case s[0] == '{':
+		if len(s) > 2 && isShellSpecialVar(s[1]) && s[2] == '}' {
+			return s[1:2], 3
+		}
+		// Scan to closing brace
+		for i := 1; i < len(s); i++ {
+			if s[i] == '}' {
+				if i == 1 {
+					return "", 2 // Bad syntax; eat "${}"
+				}
+				return s[1:i], i + 1
+			}
+		}
+		return "", 1 // Bad syntax; eat "${"
+	case isShellSpecialVar(s[0]):
+		return s[0:1], 1
+	}
+	// Scan alphanumerics.
+	var i int
+	for i = 0; i < len(s) && isAlphaNum(s[i]); i++ {
+	}
+	return s[:i], i
+}

internal/cmd/preprocessor/cmd/expand_test.go

@@ -0,0 +1,65 @@
+// Copyright 2024 The CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cmd
+
+import (
+	"testing"
+
+	"github.com/go-quicktest/qt"
+)
+
+func TestExpand(t *testing.T) {
+	mapping := func(s string) (bool, string) {
+		switch s {
+		case "BANANA":
+			return true, "apple"
+		default:
+			return false, ""
+		}
+	}
+	cases := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{
+			name:  "entire string",
+			input: "$BANANA",
+			want:  "apple",
+		},
+		{
+			name:  "double dollar", // nothing replace
+			input: "$$BANANA",
+			want:  "$$BANANA",
+		},
+		{
+			name:  "braces",
+			input: "${BANANA}",
+			want:  "apple",
+		},
+		{
+			name:  "another var", // left untouched
+			input: "$SOMETHING",
+			want:  "$SOMETHING",
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := expand(c.input, mapping)
+			qt.Assert(t, qt.Equals(got, c.want))
+		})
+	}
+}

internal/cmd/preprocessor/cmd/rootfile.go

@@ -382,7 +382,7 @@ func (rf *rootFile) buildMultistepScript() (*multiStepScript, error) {
 
 	// TODO: work out why we are not inheriting PATH from the setpriv environment
 	// whereas we do when we docker run from the command line.
-	pf("export PATH=\"/go/bin:/usr/local/go/bin:$PATH\"\n")
+	pf("export PATH=\"/cues/prerelease:/go/bin:/usr/local/go/bin:$PATH\"\n")
 
 	// exitCodeVar is the name of the "temporary" variable used to capture
 	// the exit code from a command. Named something suitably esoteric to

internal/cmd/preprocessor/cmd/script_node.go

@@ -78,6 +78,13 @@ func (s *scriptNode) validate() {
 	// Now render each statement, creating doc comments for each as we go, and
 	// gathering any non-doc comments as script node-level comments.
 
+	envSubstCUEVersions := func(v string) string {
+		return expand(v, func(vv string) (bool, string) {
+			version, ok := s.cueEnvVersions[vv]
+			return ok, version
+		})
+	}
+
 	for _, stmt := range file.Stmts {
 		cmdStmt := commandStmt{
 			stmt: stmt,
@@ -130,14 +137,14 @@ func (s *scriptNode) validate() {
 			s.errorf("%v: failed to print statement at %v: %v", s, stmt.Position, err)
 			continue
 		}
-		cmdStmt.Cmd = sb.String()
+		cmdStmt.Cmd = envSubstCUEVersions(sb.String())
 
 		sb.Reset()
 		if err := s.rf.shellPrinter.Print(&sb, &doc); err != nil {
 			s.errorf("%v: failed to print doc comment for stmt at %v: %v", s, stmt.Position, err)
 			continue
 		}
-		cmdStmt.Doc = sb.String()
+		cmdStmt.Doc = envSubstCUEVersions(sb.String())
 
 		// Now check if there are any known tag-based sanitiser directives
 		//