[Remove Nixpkgs] move concurrency code to own file (#1412)

## Summary

No logic change. Just grouping this code into its own file, ordered by
public functions/methods on top.

Also fixed this placeholder error message:
#1318 (comment)

## How was it tested?

compiles

Author: savil

internal/devpkg/narinfo_cache.go

@@ -0,0 +1,227 @@
+package devpkg
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"strings"
+	"sync"
+	"time"
+	"unicode"
+
+	"github.com/pkg/errors"
+	"go.jetpack.io/devbox/internal/boxcli/featureflag"
+	"go.jetpack.io/devbox/internal/lock"
+	"go.jetpack.io/devbox/internal/nix"
+	"go.jetpack.io/devbox/internal/vercheck"
+	"golang.org/x/sync/errgroup"
+)
+
+// BinaryCache is the store from which to fetch this package's binaries.
+// It is used as FromStore in builtins.fetchClosure.
+const BinaryCache = "https://cache.nixos.org"
+
+// isNarInfoInCache checks if the .narinfo for this package is in the `BinaryCache`.
+// This cannot be a field on the Package struct, because that struct
+// is constructed multiple times in a request (TODO: we could fix that).
+var isNarInfoInCache = struct {
+	// The key is the `Package.Raw` string.
+	status map[string]bool
+	lock   sync.RWMutex
+	// re-use httpClient to re-use the connection
+	httpClient http.Client
+}{
+	status:     map[string]bool{},
+	httpClient: http.Client{},
+}
+
+// IsInBinaryCache returns true if the package is in the binary cache.
+// ALERT: Callers must call FillNarInfoCache before calling this function.
+func (p *Package) IsInBinaryCache() (bool, error) {
+
+	if eligible, err := p.isEligibleForBinaryCache(); err != nil {
+		return false, err
+	} else if !eligible {
+		return false, nil
+	}
+
+	// Check if the narinfo is present in the binary cache
+	isNarInfoInCache.lock.RLock()
+	exists, ok := isNarInfoInCache.status[p.Raw]
+	isNarInfoInCache.lock.RUnlock()
+	if !ok {
+		return false, errors.Errorf(
+			"narInfo cache miss: %v. Call FillNarInfoCache before invoking IsInBinaryCache",
+			p.Raw,
+		)
+	}
+	return exists, nil
+}
+
+// FillNarInfoCache checks the remote binary cache for the narinfo of each
+// package in the list, and caches the result.
+// Callers of IsInBinaryCache must call this function first.
+func FillNarInfoCache(ctx context.Context, packages ...*Package) error {
+
+	// Pre-compute values read in fillNarInfoCache
+	// so they can be read from multiple go-routines without locks
+	_, err := nix.Version()
+	if err != nil {
+		return err
+	}
+	_ = nix.System()
+	for _, p := range packages {
+		_, err := p.lockfile.Resolve(p.Raw)
+		if err != nil {
+			return err
+		}
+	}
+
+	group, _ := errgroup.WithContext(ctx)
+	for _, p := range packages {
+		// If the package's NarInfo status is already known, skip it
+		isNarInfoInCache.lock.RLock()
+		_, ok := isNarInfoInCache.status[p.Raw]
+		isNarInfoInCache.lock.RUnlock()
+		if ok {
+			continue
+		}
+		pkg := p // copy the loop variable since its used in a closure below
+		group.Go(func() error {
+			err := pkg.fillNarInfoCache()
+			if err != nil {
+				// default to false if there was an error, so we don't re-try
+				isNarInfoInCache.lock.Lock()
+				isNarInfoInCache.status[pkg.Raw] = false
+				isNarInfoInCache.lock.Unlock()
+			}
+			return err
+		})
+	}
+	return group.Wait()
+}
+
+// fillNarInfoCache fills the cache value for the narinfo of this package,
+// if it is eligible for the binary cache.
+// NOTE: this must be concurrency safe.
+func (p *Package) fillNarInfoCache() error {
+	if eligible, err := p.isEligibleForBinaryCache(); err != nil {
+		return err
+	} else if !eligible {
+		return nil
+	}
+
+	sysInfo, err := p.sysInfoIfExists()
+	if err != nil {
+		return err
+	} else if sysInfo == nil {
+		return errors.New(
+			"sysInfo is nil, but should not be because" +
+				" the package is eligible for binary cache",
+		)
+	}
+
+	pathParts := newStorePathParts(sysInfo.StorePath)
+	reqURL := BinaryCache + "/" + pathParts.hash + ".narinfo"
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+	req, err := http.NewRequestWithContext(ctx, http.MethodHead, reqURL, nil)
+	if err != nil {
+		return err
+	}
+	res, err := isNarInfoInCache.httpClient.Do(req)
+	if err != nil {
+		return err
+	}
+	// read the body fully, and close it to ensure the connection is reused.
+	_, _ = io.Copy(io.Discard, res.Body)
+	defer res.Body.Close()
+
+	isNarInfoInCache.lock.Lock()
+	isNarInfoInCache.status[p.Raw] = res.StatusCode == 200
+	isNarInfoInCache.lock.Unlock()
+	return nil
+}
+
+func (p *Package) isEligibleForBinaryCache() (bool, error) {
+	sysInfo, err := p.sysInfoIfExists()
+	if err != nil {
+		return false, err
+	}
+	return sysInfo != nil, nil
+}
+
+// sysInfoIfExists returns the system info for the user's system. If the sysInfo
+// is missing, then nil is returned
+// NOTE: this is called from multiple go-routines and needs to be concurrency safe.
+// Hence, we compute nix.Version, nix.System and lockfile.Resolve prior to calling this
+// function from within a goroutine.
+func (p *Package) sysInfoIfExists() (*lock.SystemInfo, error) {
+	if !featureflag.RemoveNixpkgs.Enabled() {
+		return nil, nil
+	}
+
+	if !p.isVersioned() {
+		return nil, nil
+	}
+
+	version, err := nix.Version()
+	if err != nil {
+		return nil, err
+	}
+
+	// enable for nix >= 2.17
+	if vercheck.SemverCompare(version, "2.17.0") < 0 {
+		return nil, err
+	}
+
+	entry, err := p.lockfile.Resolve(p.Raw)
+	if err != nil {
+		return nil, err
+	}
+
+	userSystem := nix.System()
+
+	if entry.Systems == nil {
+		return nil, nil
+	}
+
+	// Check if the user's system's info is present in the lockfile
+	sysInfo, ok := entry.Systems[userSystem]
+	if !ok {
+		return nil, nil
+	}
+	return sysInfo, nil
+}
+
+// storePath are the constituent parts of
+// /nix/store/<hash>-<name>-<version>
+//
+// This is a helper struct for analyzing the string representation
+type storePathParts struct {
+	hash    string
+	name    string
+	version string
+}
+
+// newStorePathParts splits a Nix store path into its hash, name and version
+// components in the same way that Nix does.
+//
+// See https://nixos.org/manual/nix/stable/language/builtins.html#builtins-parseDrvName
+func newStorePathParts(path string) storePathParts {
+	path = strings.TrimPrefix(path, "/nix/store/")
+	// path is now <hash>-<name>-<version
+
+	hash, name := path[:32], path[33:]
+	dashIndex := 0
+	for i, r := range name {
+		if dashIndex != 0 && !unicode.IsLetter(r) {
+			return storePathParts{hash: hash, name: name[:dashIndex], version: name[i:]}
+		}
+		dashIndex = 0
+		if r == '-' {
+			dashIndex = i
+		}
+	}
+	return storePathParts{hash: hash, name: name}
+}