Per-OS CA install instructions (mitmuxd -install-ca)

Trusting the CA was previously "import ca.pem into whatever's making
the requests" with no further help. -install-ca generates the CA if
needed and prints copy-pasteable, OS-specific steps, then exits without
starting the proxy.

Deliberately instructions-only, never auto-executing anything: Linux
trust-store tooling varies enough across distros (trust vs
update-ca-trust vs update-ca-certificates) that guessing wrong and
running the wrong command unattended is worse than asking, and
installing a root CA is a system-wide trust change affecting every TLS
connection on the machine, not just mitmux's own traffic - running the
printed command themselves keeps the user in control of that.

internal/ca/install.go: InstallInstructions(goos, caPath) dispatches by
OS. Linux detects trust (p11-kit - Arch, also on Fedora) /
update-ca-trust (RHEL/Fedora/CentOS) / update-ca-certificates
(Debian/Ubuntu/Gentoo) via PATH lookup and prints whichever is actually
present, plus separate certutil/NSS instructions for Firefox/Chrome's
own certificate store (which doesn't always follow the system trust
store on Linux). macOS (security add-trusted-cert) and Windows
(certutil -addstore / Import-Certificate) are implemented from each
platform's standard documented tooling but not verified live - no
macOS/Windows machine was available to test against, unlike Linux.

commandExists is a package var (not a direct exec.LookPath call) so
tests can fake which tools are "present" and exercise every detection
branch deterministically, independent of what's actually installed on
whatever machine runs `go test`.

Verified live: built mitmuxd, ran -install-ca against a throwaway CA
dir on this (Arch Linux) machine - correctly detected `trust` and
`certutil` on PATH and printed accurate commands, confirmed the CA
files were actually generated, confirmed no proxy/daemon process was
left running (exits immediately after printing), and confirmed running
it a second time reuses the existing CA (identical file hash) rather
than regenerating.

go build/vet/gofmt/test/mod tidy all clean.

1 files changed, 84 insertions, 0 deletions

diff --git a/internal/ca/install_test.go b/internal/ca/install_test.go
new file mode 100644
index 0000000..0f01f1f
--- /dev/null
+++ b/internal/ca/install_test.go
@@ -0,0 +1,84 @@
+package ca
+
+import (
+	"strings"
+	"testing"
+)
+
+// withCommands temporarily replaces commandExists with a fake that only
+// reports the given names as present, restoring the real one after.
+func withCommands(t *testing.T, present ...string) {
+	t.Helper()
+	set := make(map[string]bool, len(present))
+	for _, p := range present {
+		set[p] = true
+	}
+	orig := commandExists
+	commandExists = func(name string) bool { return set[name] }
+	t.Cleanup(func() { commandExists = orig })
+}
+
+func TestLinuxInstructionsPrefersTrust(t *testing.T) {
+	withCommands(t, "trust", "update-ca-trust", "update-ca-certificates")
+	got := linuxInstructions("/tmp/ca.pem")
+	if !strings.Contains(got, "sudo trust anchor --store /tmp/ca.pem") {
+		t.Errorf("expected trust anchor command when trust is available, got:\n%s", got)
+	}
+}
+
+func TestLinuxInstructionsFallsBackToUpdateCaTrust(t *testing.T) {
+	withCommands(t, "update-ca-trust")
+	got := linuxInstructions("/tmp/ca.pem")
+	if !strings.Contains(got, "update-ca-trust") || strings.Contains(got, "trust anchor") {
+		t.Errorf("expected update-ca-trust path, got:\n%s", got)
+	}
+}
+
+func TestLinuxInstructionsFallsBackToUpdateCaCertificates(t *testing.T) {
+	withCommands(t, "update-ca-certificates")
+	got := linuxInstructions("/tmp/ca.pem")
+	if !strings.Contains(got, "update-ca-certificates") {
+		t.Errorf("expected update-ca-certificates path, got:\n%s", got)
+	}
+}
+
+func TestLinuxInstructionsNoneFound(t *testing.T) {
+	withCommands(t)
+	got := linuxInstructions("/tmp/ca.pem")
+	if !strings.Contains(got, "No known trust-store tool") {
+		t.Errorf("expected a no-tool-found message, got:\n%s", got)
+	}
+}
+
+func TestLinuxInstructionsCertutilPresence(t *testing.T) {
+	withCommands(t, "certutil")
+	withCert := linuxInstructions("/tmp/ca.pem")
+	if !strings.Contains(withCert, "certutil -d sql:") {
+		t.Errorf("expected certutil NSS instructions when certutil is present, got:\n%s", withCert)
+	}
+
+	withCommands(t)
+	withoutCert := linuxInstructions("/tmp/ca.pem")
+	if !strings.Contains(withoutCert, "Import manually") {
+		t.Errorf("expected manual-import fallback when certutil is absent, got:\n%s", withoutCert)
+	}
+}
+
+func TestInstallInstructionsDispatchesByOS(t *testing.T) {
+	withCommands(t)
+	tests := []struct {
+		goos string
+		want string
+	}{
+		{"linux", "trust store"},
+		{"darwin", "security add-trusted-cert"},
+		{"windows", "certutil -addstore"},
+		{"plan9", "No install steps known"},
+	}
+	for _, tt := range tests {
+		got := InstallInstructions(tt.goos, "/tmp/ca.pem")
+		if !strings.Contains(got, tt.want) {
+			t.Errorf("InstallInstructions(%q, ...) = %q, want it to contain %q", tt.goos, got, tt.want)
+		}
+	}
+}