terraform/internal/lang/funcs/string.go

// Copyright (c) HashiCorp, Inc.
// SPDX-License-Identifier: BUSL-1.1

package funcs

import (
	"fmt"
	"regexp"
	"strings"

	"github.com/hashicorp/hcl/v2"
	"github.com/hashicorp/hcl/v2/ext/customdecode"
	"github.com/hashicorp/hcl/v2/hclsyntax"
	"github.com/zclconf/go-cty/cty"
	"github.com/zclconf/go-cty/cty/convert"
	"github.com/zclconf/go-cty/cty/function"

	"github.com/hashicorp/terraform/internal/collections"
)

// StartsWithFunc constructs a function that checks if a string starts with
// a specific prefix using strings.HasPrefix
var StartsWithFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name:         "str",
			Type:         cty.String,
			AllowUnknown: true,
		},
		{
			Name: "prefix",
			Type: cty.String,
		},
	},
	Type:         function.StaticReturnType(cty.Bool),
	RefineResult: refineNotNull,
	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
		prefix := args[1].AsString()

		if !args[0].IsKnown() {
			// If the unknown value has a known prefix then we might be
			// able to still produce a known result.
			if prefix == "" {
				// The empty string is a prefix of any string.
				return cty.True, nil
			}
			if knownPrefix := args[0].Range().StringPrefix(); knownPrefix != "" {
				if strings.HasPrefix(knownPrefix, prefix) {
					return cty.True, nil
				}
				if len(knownPrefix) >= len(prefix) {
					// If the prefix we're testing is no longer than the known
					// prefix and it didn't match then the full string with
					// that same prefix can't match either.
					return cty.False, nil
				}
			}
			return cty.UnknownVal(cty.Bool), nil
		}

		str := args[0].AsString()

		if strings.HasPrefix(str, prefix) {
			return cty.True, nil
		}

		return cty.False, nil
	},
})

// EndsWithFunc constructs a function that checks if a string ends with
// a specific suffix using strings.HasSuffix
var EndsWithFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "str",
			Type: cty.String,
		},
		{
			Name: "suffix",
			Type: cty.String,
		},
	},
	Type:         function.StaticReturnType(cty.Bool),
	RefineResult: refineNotNull,
	Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
		str := args[0].AsString()
		suffix := args[1].AsString()

		if strings.HasSuffix(str, suffix) {
			return cty.True, nil
		}

		return cty.False, nil
	},
})

// ReplaceFunc constructs a function that searches a given string for another
// given substring, and replaces each occurence with a given replacement string.
var ReplaceFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "str",
			Type: cty.String,
		},
		{
			Name: "substr",
			Type: cty.String,
		},
		{
			Name: "replace",
			Type: cty.String,
		},
	},
	Type:         function.StaticReturnType(cty.String),
	RefineResult: refineNotNull,
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		str := args[0].AsString()
		substr := args[1].AsString()
		replace := args[2].AsString()

		// We search/replace using a regexp if the string is surrounded
		// in forward slashes.
		if len(substr) > 1 && substr[0] == '/' && substr[len(substr)-1] == '/' {
			re, err := regexp.Compile(substr[1 : len(substr)-1])
			if err != nil {
				return cty.UnknownVal(cty.String), err
			}

			return cty.StringVal(re.ReplaceAllString(str, replace)), nil
		}

		return cty.StringVal(strings.Replace(str, substr, replace, -1)), nil
	},
})

// StrContainsFunc searches a given string for another given substring,
// if found the function returns true, otherwise returns false.
var StrContainsFunc = function.New(&function.Spec{
	Params: []function.Parameter{
		{
			Name: "str",
			Type: cty.String,
		},
		{
			Name: "substr",
			Type: cty.String,
		},
	},
	Type: function.StaticReturnType(cty.Bool),
	Impl: func(args []cty.Value, retType cty.Type) (ret cty.Value, err error) {
		str := args[0].AsString()
		substr := args[1].AsString()

		if strings.Contains(str, substr) {
			return cty.True, nil
		}

		return cty.False, nil
	},
})

// TemplateStringFunc renders a template presented either as a literal string
// or as a reference to a string from elsewhere.
func MakeTemplateStringFunc(funcsCb func() (funcs map[string]function.Function, fsFuncs collections.Set[string], templateFuncs collections.Set[string])) function.Function {
	return function.New(&function.Spec{
		Params: []function.Parameter{
			{
				Name: "template",
				Type: customdecode.ExpressionClosureType,
			},
			{
				Name: "vars",
				Type: cty.DynamicPseudoType,
			},
		},
		Type:         function.StaticReturnType(cty.String),
		RefineResult: refineNotNull,
		Impl: func(args []cty.Value, retType cty.Type) (cty.Value, error) {
			templateClosure := customdecode.ExpressionClosureFromVal(args[0])
			varsVal := args[1]

			// Our historical experience with the hashicorp/template provider's
			// template_file data source tells us that situations where authors
			// must write a string template that generates a string template
			// cause all sorts of confusion, because the same syntax ends up
			// being evaluated in two different contexts with different variables
			// in scope, and new authors tend to be attracted to a function
			// named "template" and so miss that the language has built-in
			// support for inline template expressions.
			//
			// As a compromise to try to meet the (relatively unusual) use-cases
			// where dynamic template fetching is needed without creating an
			// attractive nuisance for those who would be better off just writing
			// a plain inline template, this function imposes constraints on how
			// the template argument may be provided and thus allows us
			// to return slightly more helpful error messages.
			//
			// The only valid way to provide the template argument is as a
			// simple, direct reference to some other value in scope that is
			// of type string:
			//       templatestring(local.greeting_template, { name = "Alex" })
			//
			// Those with more unusual desires, such as dynamically generating
			// a template at runtime by trying to concatenate template chunks
			// together, can still do such things by placing the template
			// construction expression in a separate local value and then passing
			// that local value to the template argument. But the restriction is
			// intended to intentionally add an extra "roadbump" so that
			// anyone who mistakenly thinks they need templatestring to render
			// an inline template (a common mistake for new authors with
			// template_file) will hopefully hit this roadblock and refer to
			// the function documentation to learn about the other options that
			// are probably more suitable for what they need.
			switch expr := templateClosure.Expression.(type) {
			case *hclsyntax.TemplateWrapExpr:
				// This situation occurs when someone writes an interpolation-only
				// expression as was required in Terraform v0.11 and earlier.
				// Because older versions of Terraform required this and this
				// habit has been sticky for some authors, we'll return a
				// special error message.
				return cty.UnknownVal(retType), function.NewArgErrorf(
					0, "invalid template expression: templatestring is only for rendering templates retrieved dynamically from elsewhere; to treat the inner expression as template syntax, write the reference expression directly without any template interpolation syntax",
				)
			case *hclsyntax.TemplateExpr:
				// This is the more general case of someone trying to write
				// an inline template as the argument. In this case we'll
				// distinguish between an entirely-literal template, which
				// probably suggests someone was trying to escape their template
				// for the function to consume, vs. a template with other
				// sequences that suggests someone was just trying to write
				// an inline template and so probably doesn't need to call
				// this function at all.
				literal := true
				if len(expr.Parts) != 1 {
					literal = false
				} else if _, ok := expr.Parts[0].(*hclsyntax.LiteralValueExpr); !ok {
					literal = false
				}
				if literal {
					return cty.UnknownVal(retType), function.NewArgErrorf(
						0, "invalid template expression: templatestring is only for rendering templates retrieved dynamically from elsewhere, and so does not support providing a literal template; consider using a template string expression instead",
					)
				} else {
					return cty.UnknownVal(retType), function.NewArgErrorf(
						0, "invalid template expression: templatestring is only for rendering templates retrieved dynamically from elsewhere; to render an inline template, consider using a plain template string expression",
					)
				}
			default:
				if !isValidTemplateStringExpr(expr) {
					// Someone who really does want to construct a template dynamically
					// can factor out that construction into a local value and refer
					// to it in the templatestring call, but it's not really feasible
					// to explain that clearly in a short error message so we'll deal
					// with that option on the function's documentation page instead,
					// where we can show a full example.
					return cty.UnknownVal(retType), function.NewArgErrorf(
						0, "invalid template expression: must be a direct reference to a single string from elsewhere, containing valid Terraform template syntax",
					)
				}
			}

			templateVal, diags := templateClosure.Value()
			if diags.HasErrors() {
				// With the constraints we imposed above the possible errors
				// here are pretty limited: it must be some kind of invalid
				// traversal. As usual HCL diagnostics don't make for very
				// good function errors but we've already filtered out many
				// common reasons for error here, so we should get here pretty
				// rarely.
				return cty.UnknownVal(retType), function.NewArgErrorf(
					0, "invalid template expression: %s",
					diags.Error(),
				)
			}
			if !templateVal.IsKnown() {
				// We'll need to wait until we actually know what the template is.
				return cty.UnknownVal(retType), nil
			}
			if templateVal.Type() != cty.String || templateVal.IsNull() {
				// We're being a little stricter than usual here and requiring
				// exactly a string, rather than just anything that can convert
				// to one. This is because the stringification of a number or
				// boolean value cannot be a useful template (it wouldn't have
				// any template sequences in it) and so far more likely to be
				// a mistake than actually intentional.
				return cty.UnknownVal(retType), function.NewArgErrorf(
					0, "invalid template value: a string is required",
				)
			}
			templateVal, templateMarks := templateVal.Unmark()
			templateStr := templateVal.AsString()
			expr, diags := hclsyntax.ParseTemplate([]byte(templateStr), "<templatestring argument>", hcl.Pos{Line: 1, Column: 1})
			if diags.HasErrors() {
				return cty.UnknownVal(retType), function.NewArgErrorf(
					0, "invalid template: %s",
					diags.Error(),
				)
			}

			render := makeRenderTemplateFunc(funcsCb, false)
			retVal, err := render(expr, varsVal)
			if err != nil {
				return cty.UnknownVal(retType), err
			}
			retVal, err = convert.Convert(retVal, cty.String)
			if err != nil {
				return cty.UnknownVal(retType), fmt.Errorf("invalid template result: %s", err)
			}
			return retVal.WithMarks(templateMarks), nil
		},
	})
}

func makeRenderTemplateFunc(funcsCb func() (funcs map[string]function.Function, fsFuncs collections.Set[string], templateFuncs collections.Set[string]), allowFS bool) func(expr hcl.Expression, varsVal cty.Value) (cty.Value, error) {
	return func(expr hcl.Expression, varsVal cty.Value) (cty.Value, error) {
		if varsTy := varsVal.Type(); !(varsTy.IsMapType() || varsTy.IsObjectType()) {
			return cty.DynamicVal, function.NewArgErrorf(1, "invalid vars value: must be a map") // or an object, but we don't strongly distinguish these most of the time
		}

		ctx := &hcl.EvalContext{
			Variables: varsVal.AsValueMap(),
		}

		// We require all of the variables to be valid HCL identifiers, because
		// otherwise there would be no way to refer to them in the template
		// anyway. Rejecting this here gives better feedback to the user
		// than a syntax error somewhere in the template itself.
		for n := range ctx.Variables {
			if !hclsyntax.ValidIdentifier(n) {
				// This error message intentionally doesn't describe _all_ of
				// the different permutations that are technically valid as an
				// HCL identifier, but rather focuses on what we might
				// consider to be an "idiomatic" variable name.
				return cty.DynamicVal, function.NewArgErrorf(1, "invalid template variable name %q: must start with a letter, followed by zero or more letters, digits, and underscores", n)
			}
		}

		// We'll pre-check references in the template here so we can give a
		// more specialized error message than HCL would by default, so it's
		// clearer that this problem is coming from a templatefile call.
		for _, traversal := range expr.Variables() {
			root := traversal.RootName()
			if _, ok := ctx.Variables[root]; !ok {
				return cty.DynamicVal, function.NewArgErrorf(1, "vars map does not contain key %q, referenced at %s", root, traversal[0].SourceRange())
			}
		}

		givenFuncs, fsFuncs, templateFuncs := funcsCb() // this callback indirection is to avoid chicken/egg problems
		funcs := make(map[string]function.Function, len(givenFuncs))
		for name, fn := range givenFuncs {
			plainName := strings.TrimPrefix(name, "core::")
			switch {
			case templateFuncs.Has(plainName):
				funcs[name] = function.New(&function.Spec{
					Params:   fn.Params(),
					VarParam: fn.VarParam(),
					Type: func(args []cty.Value) (cty.Type, error) {
						return cty.NilType, fmt.Errorf("cannot recursively call %s from inside another template function", plainName)
					},
				})
			case !allowFS && fsFuncs.Has(plainName):
				// Note: for now this assumes that allowFS is false only for
				// the templatestring function, and so mentions that name
				// directly in the error message.
				funcs[name] = function.New(&function.Spec{
					Params:   fn.Params(),
					VarParam: fn.VarParam(),
					Type: func(args []cty.Value) (cty.Type, error) {
						return cty.NilType, fmt.Errorf("cannot use filesystem access functions like %s in templatestring templates; consider passing the function result as a template variable instead", plainName)
					},
				})
			default:
				funcs[name] = fn
			}
		}
		ctx.Functions = funcs

		val, diags := expr.Value(ctx)
		if diags.HasErrors() {
			return cty.DynamicVal, diags
		}
		if val.IsNull() {
			return cty.DynamicVal, &hcl.Diagnostic{
				Severity: hcl.DiagError,
				Summary:  "Template result is null",
				Detail:   "The result of the template is null, which is not a valid result for a templatestring call.",
				Subject:  expr.Range().Ptr(),
			}
		}
		return val, nil
	}
}

func isValidTemplateStringExpr(expr hcl.Expression) bool {
	// Our goal with this heuristic is to be as permissive as possible with
	// allowing things that authors might try to use as references to a
	// template string defined elsewhere, while rejecting complex expressions
	// that seem like they might be trying to construct templates dynamically
	// or might have resulted from a misunderstanding that "templatestring" is
	// the only way to render a template, because someone hasn't learned
	// about template expressions yet.
	//
	// This is here only to give better feedback to folks who seem to be using
	// templatestring for something other than what it's intended for, and not
	// to block dynamic template generation altogether. Authors who have a
	// genuine need for dynamic template generation can always assert that to
	// Terraform by factoring out their dynamic generation into a local value
	// and referring to it; this rule is just a little speedbump to prompt
	// the author to consider whether there's a better way to solve their
	// problem, as opposed to just using the first solution they found.
	switch expr := expr.(type) {
	case *hclsyntax.ScopeTraversalExpr:
		// A simple static reference from the current scope is always valid.
		return true

	case *hclsyntax.RelativeTraversalExpr:
		// Relative traversals are allowed as long as they begin from
		// something that would otherwise be allowed.
		return isValidTemplateStringExpr(expr.Source)

	case *hclsyntax.IndexExpr:
		// Index expressions are allowed as long as the collection is
		// also specified using an expression that conforms to these rules.
		// The key operand is intentionally unconstrained because that
		// is a rule for how to select an element, and so doesn't represent
		// a source from which the template string is being retrieved.
		return isValidTemplateStringExpr(expr.Collection)

	case *hclsyntax.SplatExpr:
		// Splat expressions would be weird to use because they'd typically
		// return a tuple and that wouldn't be valid as a template string,
		// but we allow it here (as long as the operand would otherwise have
		// been allowed) because then we'll let the type mismatch error
		// show through, and that's likely a more helpful error message.
		return isValidTemplateStringExpr(expr.Source)

	default:
		// Nothing else is allowed.
		return false
	}
}

// Replace searches a given string for another given substring,
// and replaces all occurences with a given replacement string.
func Replace(str, substr, replace cty.Value) (cty.Value, error) {
	return ReplaceFunc.Call([]cty.Value{str, substr, replace})
}

func StrContains(str, substr cty.Value) (cty.Value, error) {
	return StrContainsFunc.Call([]cty.Value{str, substr})
}