allow using environment vars for configuration

fixes #151

README.md

@@ -287,6 +287,13 @@ you want to use a different caching implementation, it's probably easiest to
 just make a copy of `cmd/imageproxy/main.go` and customize it to fit your
 needs... it's a very simple command.
 
+### Environment Variables ###
+
+All configuration flags have equivalent environment variables of the form
+`IMAGEPROXY_$NAME`.  For example, an on-disk cache could be configured by calling
+
+    IMAGEPROXY_CACHE="/tmp/imageproxy" imageproxy
+
 ## Deploying ##
 
 In most cases, you can follow the normal procedure for building a deploying any

cmd/imageproxy/main.go

@@ -33,6 +33,7 @@ import (
 	"github.com/garyburd/redigo/redis"
 	"github.com/gregjones/httpcache/diskcache"
 	rediscache "github.com/gregjones/httpcache/redis"
+	"github.com/jamiealquiza/envy"
 	"github.com/peterbourgon/diskv"
 	"willnorris.com/go/imageproxy"
 	"willnorris.com/go/imageproxy/internal/gcscache"
@@ -60,6 +61,7 @@ func init() {
 }
 
 func main() {
+	envy.Parse("IMAGEPROXY")
 	flag.Parse()
 
 	p := imageproxy.NewProxy(nil, cache.Cache)

go.mod

@@ -17,6 +17,7 @@ require (
 	github.com/gregjones/httpcache v0.0.0-20190212212710-3befbb6ad0cc
 	github.com/grpc-ecosystem/grpc-gateway v1.8.5 // indirect
 	github.com/hashicorp/golang-lru v0.5.1 // indirect
+	github.com/jamiealquiza/envy v1.1.0
 	github.com/marstr/guid v0.0.0-20170427235115-8bdf7d1a087c // indirect
 	github.com/muesli/smartcrop v0.2.1-0.20181030220600-548bbf0c0965
 	github.com/nfnt/resize v0.0.0-20180221191011-83c6a9932646 // indirect
@@ -36,3 +37,6 @@ require (
 
 // temporary fix to https://github.com/golang/lint/issues/436 which still seems to be a problem
 replace github.com/golang/lint => github.com/golang/lint v0.0.0-20190227174305-8f45f776aaf1
+
+// local copy of envy package without cobra support
+replace github.com/jamiealquiza/envy => ./third_party/envy

third_party/envy/LICENSE

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2017 Jamie Alquiza
+
+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.

third_party/envy/README.md

@@ -0,0 +1,2 @@
+envy is a copy of https://github.com/jamiealquiza/envy without the cobra
+support.

third_party/envy/envy.go

@@ -0,0 +1,50 @@
+// Package envy automatically exposes environment
+// variables for all of your flags.
+package envy
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"strings"
+)
+
+// Parse takes a prefix string and exposes environment variables
+// for all flags in the default FlagSet (flag.CommandLine) in the
+// form of PREFIX_FLAGNAME.
+func Parse(p string) {
+	update(p, flag.CommandLine)
+}
+
+// update takes a prefix string p and *flag.FlagSet. Each flag
+// in the FlagSet is exposed as an upper case environment variable
+// prefixed with p. Any flag that was not explicitly set by a user
+// is updated to the environment variable, if set.
+func update(p string, fs *flag.FlagSet) {
+	// Build a map of explicitly set flags.
+	set := map[string]interface{}{}
+	fs.Visit(func(f *flag.Flag) {
+		set[f.Name] = nil
+	})
+
+	fs.VisitAll(func(f *flag.Flag) {
+		// Create an env var name
+		// based on the supplied prefix.
+		envVar := fmt.Sprintf("%s_%s", p, strings.ToUpper(f.Name))
+		envVar = strings.Replace(envVar, "-", "_", -1)
+
+		// Update the Flag.Value if the
+		// env var is non "".
+		if val := os.Getenv(envVar); val != "" {
+			// Update the value if it hasn't
+			// already been set.
+			if _, defined := set[f.Name]; !defined {
+				fs.Set(f.Name, val)
+			}
+		}
+
+		// Append the env var to the
+		// Flag.Usage field.
+		f.Usage = fmt.Sprintf("%s [%s]", f.Usage, envVar)
+	})
+}

third_party/envy/go.mod

@@ -0,0 +1,3 @@
+module .
+
+go 1.12