diff --git a/src/crypto/x509/cert_pool.go b/src/crypto/x509/cert_pool.go
index 2362e84688..59ab887105 100644
--- a/src/crypto/x509/cert_pool.go
+++ b/src/crypto/x509/cert_pool.go
@@ -6,6 +6,8 @@ package x509
 
 import (
 	"encoding/pem"
+	"errors"
+	"runtime"
 )
 
 // CertPool is a set of certificates.
@@ -18,12 +20,22 @@ type CertPool struct {
 // NewCertPool returns a new, empty CertPool.
 func NewCertPool() *CertPool {
 	return &CertPool{
-		make(map[string][]int),
-		make(map[string][]int),
-		nil,
+		bySubjectKeyId: make(map[string][]int),
+		byName:         make(map[string][]int),
 	}
 }
 
+// SystemCertPool returns a copy of the system cert pool.
+//
+// Any mutations to the returned pool are not written to disk and do
+// not affect any other pool.
+func SystemCertPool() (*CertPool, error) {
+	if runtime.GOOS == "windows" {
+		return nil, errors.New("crypto/x509: system root pool is not available on Windows")
+	}
+	return loadSystemRoots()
+}
+
 // findVerifiedParents attempts to find certificates in s which have signed the
 // given certificate. If any candidates were rejected then errCert will be set
 // to one of them, arbitrarily, and err will contain the reason that it was
@@ -107,10 +119,10 @@ func (s *CertPool) AppendCertsFromPEM(pemCerts []byte) (ok bool) {
 
 // Subjects returns a list of the DER-encoded subjects of
 // all of the certificates in the pool.
-func (s *CertPool) Subjects() (res [][]byte) {
-	res = make([][]byte, len(s.certs))
+func (s *CertPool) Subjects() [][]byte {
+	res := make([][]byte, len(s.certs))
 	for i, c := range s.certs {
 		res[i] = c.RawSubject
 	}
-	return
+	return res
 }
diff --git a/src/crypto/x509/root.go b/src/crypto/x509/root.go
index 8aae14e09e..787d955be4 100644
--- a/src/crypto/x509/root.go
+++ b/src/crypto/x509/root.go
@@ -7,11 +7,16 @@ package x509
 import "sync"
 
 var (
-	once        sync.Once
-	systemRoots *CertPool
+	once           sync.Once
+	systemRoots    *CertPool
+	systemRootsErr error
 )
 
 func systemRootsPool() *CertPool {
 	once.Do(initSystemRoots)
 	return systemRoots
 }
+
+func initSystemRoots() {
+	systemRoots, systemRootsErr = loadSystemRoots()
+}
diff --git a/src/crypto/x509/root_cgo_darwin.go b/src/crypto/x509/root_cgo_darwin.go
index bf4a5cdfee..f067cd7cf4 100644
--- a/src/crypto/x509/root_cgo_darwin.go
+++ b/src/crypto/x509/root_cgo_darwin.go
@@ -61,19 +61,23 @@ int FetchPEMRoots(CFDataRef *pemRoots) {
 }
 */
 import "C"
-import "unsafe"
+import (
+	"errors"
+	"unsafe"
+)
 
-func initSystemRoots() {
+func loadSystemRoots() (*CertPool, error) {
 	roots := NewCertPool()
 
 	var data C.CFDataRef = nil
 	err := C.FetchPEMRoots(&data)
 	if err == -1 {
-		return
+		// TODO: better error message
+		return nil, errors.New("crypto/x509: failed to load darwin system roots with cgo")
 	}
 
 	defer C.CFRelease(C.CFTypeRef(data))
 	buf := C.GoBytes(unsafe.Pointer(C.CFDataGetBytePtr(data)), C.int(C.CFDataGetLength(data)))
 	roots.AppendCertsFromPEM(buf)
-	systemRoots = roots
+	return roots, nil
 }
diff --git a/src/crypto/x509/root_darwin_arm_gen.go b/src/crypto/x509/root_darwin_arm_gen.go
index 5817158c33..6b373b9d53 100644
--- a/src/crypto/x509/root_darwin_arm_gen.go
+++ b/src/crypto/x509/root_darwin_arm_gen.go
@@ -184,8 +184,9 @@ const header = `
 
 package x509
 
-func initSystemRoots() {
-	systemRoots = NewCertPool()
-	systemRoots.AppendCertsFromPEM([]byte(systemRootsPEM))
+func loadSystemRoots() (*CertPool, error) {
+	p := NewCertPool()
+	p.AppendCertsFromPEM([]byte(systemRootsPEM))
+	return p
 }
 `
diff --git a/src/crypto/x509/root_darwin_armx.go b/src/crypto/x509/root_darwin_armx.go
index 37675b48a3..66b7051684 100644
--- a/src/crypto/x509/root_darwin_armx.go
+++ b/src/crypto/x509/root_darwin_armx.go
@@ -10,9 +10,10 @@
 
 package x509
 
-func initSystemRoots() {
-	systemRoots = NewCertPool()
-	systemRoots.AppendCertsFromPEM([]byte(systemRootsPEM))
+func loadSystemRoots() (*CertPool, error) {
+	p := NewCertPool()
+	p.AppendCertsFromPEM([]byte(systemRootsPEM))
+	return p
 }
 
 const systemRootsPEM = `
diff --git a/src/crypto/x509/root_nocgo_darwin.go b/src/crypto/x509/root_nocgo_darwin.go
index d00e257662..2ac4666aff 100644
--- a/src/crypto/x509/root_nocgo_darwin.go
+++ b/src/crypto/x509/root_nocgo_darwin.go
@@ -6,6 +6,6 @@
 
 package x509
 
-func initSystemRoots() {
-	systemRoots, _ = execSecurityRoots()
+func loadSystemRoots() (*CertPool, error) {
+	return execSecurityRoots()
 }
diff --git a/src/crypto/x509/root_plan9.go b/src/crypto/x509/root_plan9.go
index 9965caadee..ebeb7dfccd 100644
--- a/src/crypto/x509/root_plan9.go
+++ b/src/crypto/x509/root_plan9.go
@@ -6,7 +6,10 @@
 
 package x509
 
-import "io/ioutil"
+import (
+	"io/ioutil"
+	"os"
+)
 
 // Possible certificate files; stop after finding one.
 var certFiles = []string{
@@ -17,17 +20,18 @@ func (c *Certificate) systemVerify(opts *VerifyOptions) (chains [][]*Certificate
 	return nil, nil
 }
 
-func initSystemRoots() {
+func loadSystemRoots() (*CertPool, error) {
 	roots := NewCertPool()
+	var bestErr error
 	for _, file := range certFiles {
 		data, err := ioutil.ReadFile(file)
 		if err == nil {
 			roots.AppendCertsFromPEM(data)
-			systemRoots = roots
-			return
+			return roots, nil
+		}
+		if bestErr == nil || (os.IsNotExist(bestErr) && !os.IsNotExist(err)) {
+			bestErr = err
 		}
 	}
-
-	// All of the files failed to load. systemRoots will be nil which will
-	// trigger a specific error at verification time.
+	return nil, bestErr
 }
diff --git a/src/crypto/x509/root_unix.go b/src/crypto/x509/root_unix.go
index 9f06f9dabb..7bcb3d63d1 100644
--- a/src/crypto/x509/root_unix.go
+++ b/src/crypto/x509/root_unix.go
@@ -6,7 +6,10 @@
 
 package x509
 
-import "io/ioutil"
+import (
+	"io/ioutil"
+	"os"
+)
 
 // Possible directories with certificate files; stop after successfully
 // reading at least one file from a directory.
@@ -19,20 +22,26 @@ func (c *Certificate) systemVerify(opts *VerifyOptions) (chains [][]*Certificate
 	return nil, nil
 }
 
-func initSystemRoots() {
+func loadSystemRoots() (*CertPool, error) {
 	roots := NewCertPool()
+	var firstErr error
 	for _, file := range certFiles {
 		data, err := ioutil.ReadFile(file)
 		if err == nil {
 			roots.AppendCertsFromPEM(data)
-			systemRoots = roots
-			return
+			return roots, nil
+		}
+		if firstErr == nil && !os.IsNotExist(err) {
+			firstErr = err
 		}
 	}
 
 	for _, directory := range certDirectories {
 		fis, err := ioutil.ReadDir(directory)
 		if err != nil {
+			if firstErr == nil && !os.IsNotExist(err) {
+				firstErr = err
+			}
 			continue
 		}
 		rootsAdded := false
@@ -43,11 +52,9 @@ func initSystemRoots() {
 			}
 		}
 		if rootsAdded {
-			systemRoots = roots
-			return
+			return roots, nil
 		}
 	}
 
-	// All of the files failed to load. systemRoots will be nil which will
-	// trigger a specific error at verification time.
+	return nil, firstErr
 }
diff --git a/src/crypto/x509/root_windows.go b/src/crypto/x509/root_windows.go
index 51c3be3fa4..392c869012 100644
--- a/src/crypto/x509/root_windows.go
+++ b/src/crypto/x509/root_windows.go
@@ -225,5 +225,4 @@ func (c *Certificate) systemVerify(opts *VerifyOptions) (chains [][]*Certificate
 	return chains, nil
 }
 
-func initSystemRoots() {
-}
+func loadSystemRoots() (*CertPool, error) { return nil, nil }
diff --git a/src/crypto/x509/verify.go b/src/crypto/x509/verify.go
index d3b62d174c..85c083fbb2 100644
--- a/src/crypto/x509/verify.go
+++ b/src/crypto/x509/verify.go
@@ -117,10 +117,16 @@ func (e UnknownAuthorityError) Error() string {
 }
 
 // SystemRootsError results when we fail to load the system root certificates.
-type SystemRootsError struct{}
+type SystemRootsError struct {
+	Err error
+}
 
-func (SystemRootsError) Error() string {
-	return "x509: failed to load system roots and no roots provided"
+func (se SystemRootsError) Error() string {
+	msg := "x509: failed to load system roots and no roots provided"
+	if se.Err != nil {
+		return msg + "; " + se.Err.Error()
+	}
+	return msg
 }
 
 // errNotParsed is returned when a certificate without ASN.1 contents is
@@ -240,7 +246,7 @@ func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err e
 	if opts.Roots == nil {
 		opts.Roots = systemRootsPool()
 		if opts.Roots == nil {
-			return nil, SystemRootsError{}
+			return nil, SystemRootsError{systemRootsErr}
 		}
 	}