Add first iteration and test suite

.drone.yml

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+---
+kind: pipeline
+type: docker
+name: default
+
+steps:
+- name: go-1.18
+  image: golang:1.18
+  commands:
+    - go install honnef.co/go/tools/cmd/staticcheck@latest
+    - go build ./...
+    - go vet ./...
+    - staticcheck ./...
+    - go test -bench=. -benchmem -cover ./...

README.md

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+# envfilesubst
+
+envfilesubst is a variation of gettext's envsubst, with a different modus
+operandi.
+
+Firstly, instead of reading the current environment, it instead parses a file
+in "envfile" format.
+
+Secondly, it will read from standard input and replace all variable references
+that can be found in the envfile. If variables are not explicitly mentioned in
+the envfile, the references will be left intact (instead of replacing them with
+an empty string).
+
+## Installation
+
+```
+go install git.netflux.io/rob/envfilesubst@latest
+```
+
+## Usage
+
+Given an envfile:
+
+```
+FOO=bar
+X=1
+```
+
+Then:
+
+```
+echo "FOO is $FOO and X is ${X}. I don't know $BAR." | envfilesubst -f myenvfile
+```
+
+The output is:
+
+```
+FOO is bar and X is 1. I don't know $BAR.
+```
+
+## License
+
+Copyright © 2022 Rob Watson.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the “Software”), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

go.mod

@@ -0,0 +1,14 @@
+module git.netflux.io/rob/envfilesubst
+
+go 1.18
+
+require (
+	github.com/hashicorp/go-envparse v0.0.0-20200406174449-d9cfd743a15e
+	github.com/stretchr/testify v1.7.1
+)
+
+require (
+	github.com/davecgh/go-spew v1.1.0 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c // indirect
+)

go.sum

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+github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
+github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/hashicorp/go-envparse v0.0.0-20200406174449-d9cfd743a15e h1:v1d9+AJMP6i4p8BSKNU0InuvmIAdZjQLNN19V86AG4Q=
+github.com/hashicorp/go-envparse v0.0.0-20200406174449-d9cfd743a15e/go.mod h1:/NlxCzN2D4C4L2uDE6ux/h6jM+n98VFQM14nnCIfHJU=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/testify v1.7.1 h1:5TQK59W5E3v0r2duFAb7P95B6hEeOyEnHRa8MjYSMTY=
+github.com/stretchr/testify v1.7.1/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c h1:dUUwHk2QECo/6vqA44rthZ8ie2QXMNeKRTHCNY2nXvo=
+gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

main.go

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+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+
+	"git.netflux.io/rob/envfilesubst/scanner"
+)
+
+const version = "0.1"
+
+func main() {
+	var (
+		path         string
+		printVersion bool
+	)
+
+	flag.StringVar(&path, "f", "", "The envfile to read from")
+	flag.BoolVar(&printVersion, "version", false, "Print version and exit")
+	flag.Parse()
+
+	if printVersion {
+		fmt.Fprintf(os.Stderr, "envfilesubst version %s\n", version)
+		os.Exit(0)
+	}
+
+	if path == "" {
+		fmt.Fprint(os.Stderr, "envfilesubst reads an input from stdin and pipes it to stdout, replacing $ENV_VAR or ${ENV_VAR} occurrences that can be found in the provided envfile.\n\n")
+		fmt.Fprint(os.Stderr, "Variables that cannot be found in envfile are left unchanged.\n\n")
+		fmt.Fprint(os.Stderr, "Usage:\n")
+		flag.PrintDefaults()
+		os.Exit(1)
+	}
+
+	envfile, err := os.Open(path)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error: %v", err)
+		os.Exit(1)
+	}
+
+	scanner := scanner.New(os.Stdout, os.Stdin, envfile)
+	if err := scanner.Scan(); err != nil {
+		fmt.Fprintf(os.Stderr, "Error: %v", err)
+		os.Exit(1)
+	}
+}

scanner/scanner.go

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+package scanner
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"regexp"
+	"strings"
+
+	"github.com/hashicorp/go-envparse"
+)
+
+var envvarRegex = regexp.MustCompile(`\$\{?([A-Z][A-Z0-9_]*)\}?`)
+
+type Scanner struct {
+	envfile, r io.Reader
+	w          io.Writer
+}
+
+// New returns a new Scanner with the provided arguments.
+func New(w io.Writer, r io.Reader, envfile io.Reader) *Scanner {
+	return &Scanner{
+		w:       w,
+		r:       r,
+		envfile: envfile,
+	}
+}
+
+const nl = "\n"
+
+func (s *Scanner) Scan() error {
+	vars, err := envparse.Parse(s.envfile)
+	if err != nil {
+		return fmt.Errorf("error parsing envfile: %v", err)
+	}
+
+	scanner := bufio.NewScanner(s.r)
+	for scanner.Scan() {
+		text := scanner.Text()
+		matchIndices := envvarRegex.FindAllStringSubmatchIndex(text, -1)
+
+		var sb strings.Builder
+		var c int
+		for _, idx := range matchIndices {
+			m1, m2, n1, n2 := idx[0], idx[1], idx[2], idx[3]
+			writeString(&sb, text[c:m1])
+			c = m2
+
+			name := text[n1:n2]
+			if val, ok := vars[name]; ok {
+				writeString(&sb, val)
+			} else {
+				writeString(&sb, text[m1:m2])
+			}
+		}
+
+		writeString(&sb, text[c:])
+		writeString(&sb, nl)
+
+		if _, err := s.w.Write([]byte(sb.String())); err != nil {
+			return fmt.Errorf("error writing to output: %v", err)
+		}
+	}
+
+	if err := scanner.Err(); err != nil {
+		return fmt.Errorf("error reading input: %v", err)
+	}
+
+	return nil
+}
+
+// writeString writes a string to a StringBuilder, discarding the result and
+// (non-existent) error.
+func writeString(sb *strings.Builder, s string) {
+	_, _ = sb.WriteString(s)
+}

scanner/scanner_test.go

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+package scanner_test
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"git.netflux.io/rob/envfilesubst/scanner"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestScanner(t *testing.T) {
+	envfile := `FOO=bar
+BAR=baz
+BAZ=123
+FOO_BAR=true
+QUUX1=2
+`
+
+	testCases := []struct {
+		name       string
+		envfile    string
+		input      string
+		wantOutput string
+		wantError  string
+	}{
+		{
+			name:       "single variable",
+			envfile:    envfile,
+			input:      "$FOO",
+			wantOutput: "bar\n",
+		},
+		{
+			name:       "input with prefix text",
+			envfile:    envfile,
+			input:      "baz $FOO",
+			wantOutput: "baz bar\n",
+		},
+		{
+			name:       "input with suffix text",
+			envfile:    envfile,
+			input:      "$FOO baz",
+			wantOutput: "bar baz\n",
+		},
+		{
+			name:       "input with prefix and suffix text",
+			envfile:    envfile,
+			input:      "baz $FOO qux",
+			wantOutput: "baz bar qux\n",
+		},
+		{
+			name:       "input with prefix and suffix text and whitespace",
+			envfile:    envfile,
+			input:      "\tbaz $FOO qux  ",
+			wantOutput: "\tbaz bar qux  \n",
+		},
+		{
+			name:       "single variable with curly brackets",
+			envfile:    envfile,
+			input:      "${FOO}",
+			wantOutput: "bar\n",
+		},
+		{
+			name:       "multiple variables on a single line",
+			envfile:    envfile,
+			input:      "qux ${FOO} quxx $BAR $BAZ",
+			wantOutput: "qux bar quxx baz 123\n",
+		},
+		{
+			name:       "non-existent variable",
+			envfile:    envfile,
+			input:      "$NOPE",
+			wantOutput: "$NOPE\n",
+		},
+		{
+			name:       "non-existent variable with curly brackets",
+			envfile:    envfile,
+			input:      "${NOPE}",
+			wantOutput: "${NOPE}\n",
+		},
+		{
+			name:       "multiple variables including non-existent",
+			envfile:    envfile,
+			input:      "$FOO $BAR $NOPE $BAZ",
+			wantOutput: "bar baz $NOPE 123\n",
+		},
+		{
+			name:       "variable name with an underscore",
+			envfile:    envfile,
+			input:      "$FOO_BAR is true",
+			wantOutput: "true is true\n",
+		},
+		{
+			name:       "variable name with a number",
+			envfile:    envfile,
+			input:      "$QUUX1 + ${QUUX1} = 4",
+			wantOutput: "2 + 2 = 4\n",
+		},
+		{
+			name:    "multiline input ending with newline",
+			envfile: envfile,
+			input: `---
+metadata:
+	name: "$FOO"
+	labels:
+		bar: "$BAR"
+		baz: "$BAZ"
+`,
+			wantOutput: `---
+metadata:
+	name: "bar"
+	labels:
+		bar: "baz"
+		baz: "123"
+`,
+		},
+		{
+			name:    "multiline input not ending with newline",
+			envfile: envfile,
+			input: `---
+metadata:
+	name: "$FOO"
+	labels:
+		bar: "$BAR"
+		baz: "$BAZ"`,
+			wantOutput: `---
+metadata:
+	name: "bar"
+	labels:
+		bar: "baz"
+		baz: "123"
+`,
+		},
+		{
+			name:       "empty string",
+			envfile:    envfile,
+			input:      "",
+			wantOutput: "",
+		},
+		{
+			name:       "multiline with only newlines",
+			envfile:    envfile,
+			input:      "\n\n\n\n\n",
+			wantOutput: "\n\n\n\n\n",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			envfile := strings.NewReader(strings.TrimSpace(tc.envfile))
+			input := strings.NewReader(tc.input)
+			var output bytes.Buffer
+
+			scanner := scanner.New(&output, input, envfile)
+
+			err := scanner.Scan()
+
+			if tc.wantError == "" {
+				assert.NoError(t, err)
+				assert.Equal(t, tc.wantOutput, output.String())
+			} else {
+				assert.EqualError(t, err, tc.wantError)
+			}
+		})
+	}
+}