refactor(context): Implement more explicit merging of variables

The hierarchy for loading variables was previously not expressed
explicitly.

This commit refactors the logic for merging variables to explicitly
set the different layers of variables as values on the context object
and merge them for each resource set in `mergeContextValues`.

1 files changed, 56 insertions, 30 deletions

diff --git a/context/context.go b/context/context.go
index 1a2e5c88cc09..42ad2e8c18c8 100644
--- a/context/context.go
+++ b/context/context.go
@@ -43,11 +43,17 @@ type Context struct {
 	Global map[string]interface{} `json:"global"`
 
 	// File names of YAML or JSON files including extra variables that should be globally accessible
-	VariableImports []string `json:"import"`
+	VariableImportFiles []string `json:"import"`
 
 	// The resource sets to include in this context
 	ResourceSets []ResourceSet `json:"include"`
 
+	// Variables imported from additional files
+	ImportedVars map[string]interface{}
+
+	// Explicitly set variables (via `--var`) that should override all others
+	ExplicitVars map[string]interface{}
+
 	// This field represents the absolute path to the context base directory and should not be manually specified.
 	BaseDir string
 }
@@ -57,41 +63,59 @@ func contextLoadingError(filename string, cause error) error {
 }
 
 // Attempt to load and deserialise a Context from the specified file.
-func LoadContextFromFile(filename string) (*Context, error) {
-	var c Context
-	err := util.LoadJsonOrYaml(filename, &c)
+func LoadContext(filename string, explicitVars *[]string) (*Context, error) {
+	var ctx Context
+	err := util.LoadJsonOrYaml(filename, &ctx)
 
 	if err != nil {
 		return nil, contextLoadingError(filename, err)
 	}
 
-	c.ResourceSets = flattenPrepareResourceSetPaths(&c.ResourceSets)
-	c.BaseDir = path.Dir(filename)
-	c.ResourceSets = loadAllDefaultValues(&c)
+	ctx.BaseDir = path.Dir(filename)
 
-	err = c.loadImportedVariables()
+	// Prepare the resource sets by resolving parents etc.
+	ctx.ResourceSets = flattenPrepareResourceSetPaths(&ctx.ResourceSets)
+
+	// Add variables explicitly specified on the command line
+	ctx.ExplicitVars, err = loadExplicitVars(explicitVars)
+	if err != nil {
+		return nil, fmt.Errorf("Error setting explicit variables: %v\n", err)
+	}
+
+	// Add variables loaded from import files
+	ctx.ImportedVars, err = ctx.loadImportedVariables()
 	if err != nil {
 		return nil, contextLoadingError(filename, err)
 	}
 
-	return &c, nil
+	// Merge variables (explicit > import > include >  global > default)
+	ctx.ResourceSets = ctx.mergeContextValues()
+
+	if err != nil {
+		return nil, contextLoadingError(filename, err)
+	}
+
+	return &ctx, nil
 }
 
-// Kontemplate supports specifying additional variable files with the `import` keyword. This function loads those
-// variable files and merges them together with the context's other global variables.
-func (ctx *Context) loadImportedVariables() error {
-	for _, file := range ctx.VariableImports {
+// Kontemplate supports specifying additional variable files with the
+// `import` keyword. This function loads those variable files and
+// merges them together with the context's other global variables.
+func (ctx *Context) loadImportedVariables() (map[string]interface{}, error) {
+	allImportedVars := make(map[string]interface{})
+
+	for _, file := range ctx.VariableImportFiles {
 		var importedVars map[string]interface{}
 		err := util.LoadJsonOrYaml(path.Join(ctx.BaseDir, file), &importedVars)
 
 		if err != nil {
-			return err
+			return nil, err
 		}
 
-		ctx.Global = *util.Merge(&ctx.Global, &importedVars)
+		allImportedVars = *util.Merge(&allImportedVars, &importedVars)
 	}
 
-	return nil
+	return allImportedVars, nil
 }
 
 // Correctly prepares the file paths for resource sets by inferring implicit paths and flattening resource set
@@ -128,11 +152,16 @@ func flattenPrepareResourceSetPaths(rs *[]ResourceSet) []ResourceSet {
 	return flattened
 }
 
-func loadAllDefaultValues(c *Context) []ResourceSet {
-	updated := make([]ResourceSet, len(c.ResourceSets))
+// Merges the context and resource set variables according in the
+// desired precedence order.
+func (ctx *Context) mergeContextValues() []ResourceSet {
+	updated := make([]ResourceSet, len(ctx.ResourceSets))
 
-	for i, rs := range c.ResourceSets {
-		merged := loadDefaultValues(&rs, c)
+	for i, rs := range ctx.ResourceSets {
+		merged := loadDefaultValues(&rs, ctx)
+		merged = util.Merge(&ctx.Global, merged)
+		merged = util.Merge(merged, &ctx.ImportedVars)
+		merged = util.Merge(merged, &ctx.ExplicitVars)
 		rs.Values = *merged
 		updated[i] = rs
 	}
@@ -160,22 +189,19 @@ func loadDefaultValues(rs *ResourceSet, c *Context) *map[string]interface{} {
 	return &rs.Values
 }
 
-// New variables can be defined or default values overridden with command line arguments when executing kontemplate.
-func (ctx *Context) SetVariablesFromArguments(vars *[]string) error {
-	// Resource set files might not have defined any global variables, if so we have to
-	// create that a map before potentially writing variables into it
-	if ctx.Global == nil {
-		ctx.Global = make(map[string]interface{}, len(*vars))
-	}
+// Prepares the variables specified explicitly via `--var` when
+// executing kontemplate for adding to the context.
+func loadExplicitVars(vars *[]string) (map[string]interface{}, error) {
+	explicitVars := make(map[string]interface{}, len(*vars))
 
 	for _, v := range *vars {
 		varParts := strings.Split(v, "=")
 		if len(varParts) != 2 {
-			return fmt.Errorf(`invalid explicit variable provided (%s), name and value should be divided with "="`, v)
+			return nil, fmt.Errorf(`invalid explicit variable provided (%s), name and value should be separated with "="`, v)
 		}
 
-		ctx.Global[varParts[0]] = varParts[1]
+		explicitVars[varParts[0]] = varParts[1]
 	}
 
-	return nil
+	return explicitVars, nil
 }