feat: Merge multiple docs inside a single helmfile.yaml (roboll#270)

Resolves roboll#254

docs/writing-helmfile.md

@@ -0,0 +1,72 @@
+# The Helmfile Best Practices Guide
+
+This guide covers the Helmfile’s considered patterns for writing advanced helmfiles. It focuses on how helmfile should be structured and executed.
+
+## Layering
+
+You may occasionally end up with many helmfiles that shares common parts like which repositories to use, and whichi release to be bundled by default.
+
+Use Layering to extract te common parts into a dedicated *library helmfile*s, so that each helmfile becomes DRY.
+
+Let's assume that your `helmfile.yaml` looks like:
+
+```
+{ readFile "commons.yaml" }}
+---
+{{ readFile "environments.yaml" }}
+---
+releases:
+- name: myapp
+  chart: mychart
+```
+
+Whereas `commons.yaml` contained a monitoring agent:
+
+```yaml
+releases:
+- name: metricbaet
+  chart: stable/metricbeat
+```
+
+And `environments.yaml` contained well-known environments:
+
+```yaml
+environments:
+  development:
+  production:
+```
+
+At run time, template expressions in your `helmfile.yaml` are executed:
+
+```yaml
+releases:
+- name: metricbaet
+  chart: stable/metricbeat
+---
+environments:
+  development:
+  production:
+---
+releases:
+- name: myapp
+  chart: mychart
+```
+
+Resulting YAML documents are merged in the order of occurrence,
+so that your `helmfile.yaml` becomes:
+
+```yaml
+environments:
+  development:
+  production:
+
+releases:
+- name: metricbaet
+  chart: stable/metricbeat
+- name: myapp
+  chart: mychart
+```
+
+Great!
+
+Now, repeat the above steps for each your `helmfile.yaml`, so that all your helmfiles becomes DRY.

state/create.go

@@ -1,13 +1,15 @@
 package state
 
 import (
+	"bytes"
 	"fmt"
 	"github.com/imdario/mergo"
 	"github.com/roboll/helmfile/environment"
 	"github.com/roboll/helmfile/helmexec"
 	"github.com/roboll/helmfile/valuesfile"
 	"go.uber.org/zap"
 	"gopkg.in/yaml.v2"
+	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -64,16 +66,32 @@ func (c *creator) CreateFromYaml(content []byte, file string, env string) (*Helm
 	if err != nil {
 		return nil, &StateLoadError{fmt.Sprintf("failed to read %s", file), err}
 	}
+	state.FilePath = file
 	state.basePath = basePath
 
-	unmarshal := yaml.UnmarshalStrict
+	decoder := yaml.NewDecoder(bytes.NewReader(content))
 	if !c.Strict {
-		unmarshal = yaml.Unmarshal
+		decoder.SetStrict(false)
+	} else {
+		decoder.SetStrict(true)
 	}
-	if err := unmarshal(content, &state); err != nil {
-		return nil, &StateLoadError{fmt.Sprintf("failed to read %s", file), err}
+	i := 0
+	for {
+		i++
+
+		var intermediate HelmState
+
+		err := decoder.Decode(&intermediate)
+		if err == io.EOF {
+			break
+		} else if err != nil {
+			return nil, &StateLoadError{fmt.Sprintf("failed to read %s: reading document at index %d", file, i), err}
+		}
+
+		if err := mergo.Merge(&state, &intermediate, mergo.WithAppendSlice); err != nil {
+			return nil, &StateLoadError{fmt.Sprintf("failed to read %s: merging document at index %d", file, i), err}
+		}
 	}
-	state.FilePath = file
 
 	if len(state.DeprecatedReleases) > 0 {
 		if len(state.Releases) > 0 {