[feature] S3 support (#674)

* feat: vendor minio client

* feat: introduce storage package with s3 support

* feat: serve s3 files directly

this saves a lot of bandwith as the files are fetched from the object
store directly

* fix: use explicit local storage in tests

* feat: integrate s3 storage with the main server

* fix: add s3 config to cli tests

* docs: explicitly set values in example config

also adds license header to the storage package

* fix: use better http status code on s3 redirect

HTTP 302 Found is the best fit, as it signifies that the resource
requested was found but not under its presumed URL

307/TemporaryRedirect would mean that this resource is usually located
here, not in this case

303/SeeOther indicates that the redirection does not link to the
requested resource but to another page

* refactor: use context in storage driver interface

vendor/github.com/mitchellh/go-homedir/LICENSE

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2013 Mitchell Hashimoto
+
+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.

vendor/github.com/mitchellh/go-homedir/README.md

@@ -0,0 +1,14 @@
+# go-homedir
+
+This is a Go library for detecting the user's home directory without
+the use of cgo, so the library can be used in cross-compilation environments.
+
+Usage is incredibly simple, just call `homedir.Dir()` to get the home directory
+for a user, and `homedir.Expand()` to expand the `~` in a path to the home
+directory.
+
+**Why not just use `os/user`?** The built-in `os/user` package requires
+cgo on Darwin systems. This means that any Go code that uses that package
+cannot cross compile. But 99% of the time the use for `os/user` is just to
+retrieve the home directory, which we can do for the current user without
+cgo. This library does that, enabling cross-compilation.

vendor/github.com/mitchellh/go-homedir/homedir.go

@@ -0,0 +1,167 @@
+package homedir
+
+import (
+	"bytes"
+	"errors"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+)
+
+// DisableCache will disable caching of the home directory. Caching is enabled
+// by default.
+var DisableCache bool
+
+var homedirCache string
+var cacheLock sync.RWMutex
+
+// Dir returns the home directory for the executing user.
+//
+// This uses an OS-specific method for discovering the home directory.
+// An error is returned if a home directory cannot be detected.
+func Dir() (string, error) {
+	if !DisableCache {
+		cacheLock.RLock()
+		cached := homedirCache
+		cacheLock.RUnlock()
+		if cached != "" {
+			return cached, nil
+		}
+	}
+
+	cacheLock.Lock()
+	defer cacheLock.Unlock()
+
+	var result string
+	var err error
+	if runtime.GOOS == "windows" {
+		result, err = dirWindows()
+	} else {
+		// Unix-like system, so just assume Unix
+		result, err = dirUnix()
+	}
+
+	if err != nil {
+		return "", err
+	}
+	homedirCache = result
+	return result, nil
+}
+
+// Expand expands the path to include the home directory if the path
+// is prefixed with `~`. If it isn't prefixed with `~`, the path is
+// returned as-is.
+func Expand(path string) (string, error) {
+	if len(path) == 0 {
+		return path, nil
+	}
+
+	if path[0] != '~' {
+		return path, nil
+	}
+
+	if len(path) > 1 && path[1] != '/' && path[1] != '\\' {
+		return "", errors.New("cannot expand user-specific home dir")
+	}
+
+	dir, err := Dir()
+	if err != nil {
+		return "", err
+	}
+
+	return filepath.Join(dir, path[1:]), nil
+}
+
+// Reset clears the cache, forcing the next call to Dir to re-detect
+// the home directory. This generally never has to be called, but can be
+// useful in tests if you're modifying the home directory via the HOME
+// env var or something.
+func Reset() {
+	cacheLock.Lock()
+	defer cacheLock.Unlock()
+	homedirCache = ""
+}
+
+func dirUnix() (string, error) {
+	homeEnv := "HOME"
+	if runtime.GOOS == "plan9" {
+		// On plan9, env vars are lowercase.
+		homeEnv = "home"
+	}
+
+	// First prefer the HOME environmental variable
+	if home := os.Getenv(homeEnv); home != "" {
+		return home, nil
+	}
+
+	var stdout bytes.Buffer
+
+	// If that fails, try OS specific commands
+	if runtime.GOOS == "darwin" {
+		cmd := exec.Command("sh", "-c", `dscl -q . -read /Users/"$(whoami)" NFSHomeDirectory | sed 's/^[^ ]*: //'`)
+		cmd.Stdout = &stdout
+		if err := cmd.Run(); err == nil {
+			result := strings.TrimSpace(stdout.String())
+			if result != "" {
+				return result, nil
+			}
+		}
+	} else {
+		cmd := exec.Command("getent", "passwd", strconv.Itoa(os.Getuid()))
+		cmd.Stdout = &stdout
+		if err := cmd.Run(); err != nil {
+			// If the error is ErrNotFound, we ignore it. Otherwise, return it.
+			if err != exec.ErrNotFound {
+				return "", err
+			}
+		} else {
+			if passwd := strings.TrimSpace(stdout.String()); passwd != "" {
+				// username:password:uid:gid:gecos:home:shell
+				passwdParts := strings.SplitN(passwd, ":", 7)
+				if len(passwdParts) > 5 {
+					return passwdParts[5], nil
+				}
+			}
+		}
+	}
+
+	// If all else fails, try the shell
+	stdout.Reset()
+	cmd := exec.Command("sh", "-c", "cd && pwd")
+	cmd.Stdout = &stdout
+	if err := cmd.Run(); err != nil {
+		return "", err
+	}
+
+	result := strings.TrimSpace(stdout.String())
+	if result == "" {
+		return "", errors.New("blank output when reading home directory")
+	}
+
+	return result, nil
+}
+
+func dirWindows() (string, error) {
+	// First prefer the HOME environmental variable
+	if home := os.Getenv("HOME"); home != "" {
+		return home, nil
+	}
+
+	// Prefer standard environment variable USERPROFILE
+	if home := os.Getenv("USERPROFILE"); home != "" {
+		return home, nil
+	}
+
+	drive := os.Getenv("HOMEDRIVE")
+	path := os.Getenv("HOMEPATH")
+	home := drive + path
+	if drive == "" || path == "" {
+		return "", errors.New("HOMEDRIVE, HOMEPATH, or USERPROFILE are blank")
+	}
+
+	return home, nil
+}