fix(errors): say when the server's certificate is the problem

sheaf/app/src/main/java/systems/lupine/sheaf/util/ErrorMessages.kt

@@ -5,9 +5,15 @@ import java.io.IOException
 import java.net.ConnectException
 import java.net.SocketTimeoutException
 import java.net.UnknownHostException
+import java.security.cert.CertPathValidatorException
+import java.security.cert.CertificateException
+import java.security.cert.CertificateExpiredException
+import java.security.cert.CertificateNotYetValidException
+import javax.net.ssl.SSLHandshakeException
+import javax.net.ssl.SSLPeerUnverifiedException
 
 fun Throwable.toUserMessage(fallback: String = "Something went wrong — please try again"): String =
-    when (this) {
+    certificateProblem() ?: when (this) {
         is HttpException -> {
             val body = runCatching { response()?.errorBody()?.string() ?: "" }.getOrDefault("")
             if (isCloudflareResponse(body)) {
@@ -23,6 +29,27 @@ fun Throwable.toUserMessage(fallback: String = "Something went wrong — please
         else -> fallback
     }
 
+// These are IOExceptions too, so without this they read as "check your
+// connection", which sends people debugging their network. Release builds only;
+// debug builds accept any certificate.
+private fun Throwable.certificateProblem(): String? {
+    if (this is SSLPeerUnverifiedException) {
+        return "The server's certificate doesn't match this address. Use the address the " +
+            "certificate was issued for"
+    }
+    if (this !is SSLHandshakeException) return null
+    val causes = generateSequence(cause) { it.cause }
+    return when {
+        causes.any { it is CertificateExpiredException || it is CertificateNotYetValidException } ->
+            "The server's certificate has expired or isn't valid yet. Check this device's date " +
+                "and time, or renew the certificate"
+        causes.any { it is CertPathValidatorException || it is CertificateException } ->
+            "The server's certificate isn't trusted. Sheaf doesn't use certificates installed " +
+                "on this device, so the server needs one from a public certificate authority"
+        else -> null
+    }
+}
+
 private fun isCloudflareResponse(body: String): Boolean {
     val lower = body.lowercase()
     return "cloudflare" in lower || "cf-ray" in lower

sheaf/app/src/test/java/systems/lupine/sheaf/util/ErrorMessagesTest.kt

@@ -8,6 +8,11 @@ import java.io.IOException
 import java.net.ConnectException
 import java.net.SocketTimeoutException
 import java.net.UnknownHostException
+import java.security.cert.CertPathValidatorException
+import java.security.cert.CertificateException
+import java.security.cert.CertificateExpiredException
+import javax.net.ssl.SSLHandshakeException
+import javax.net.ssl.SSLPeerUnverifiedException
 import kotlin.test.Test
 import kotlin.test.assertEquals
 import kotlin.test.assertTrue
@@ -46,6 +51,54 @@ class ErrorMessagesTest {
         )
     }
 
+    // Shaped like what Android throws: the handshake exception wraps the
+    // validator's verdict a level or two down.
+    private fun handshake(cause: Throwable) =
+        SSLHandshakeException("handshake failed").apply { initCause(cause) }
+
+    @Test
+    fun `untrusted certificate says so instead of blaming the network`() {
+        val e = handshake(
+            CertificateException(CertPathValidatorException("Trust anchor for certification path not found.")),
+        )
+        assertEquals(
+            "The server's certificate isn't trusted. Sheaf doesn't use certificates installed " +
+                "on this device, so the server needs one from a public certificate authority",
+            e.toUserMessage(),
+        )
+    }
+
+    @Test
+    fun `expired certificate is not reported as untrusted`() {
+        val e = handshake(
+            CertificateException(
+                CertPathValidatorException("timestamp check failed", CertificateExpiredException()),
+            ),
+        )
+        assertEquals(
+            "The server's certificate has expired or isn't valid yet. Check this device's date " +
+                "and time, or renew the certificate",
+            e.toUserMessage(),
+        )
+    }
+
+    @Test
+    fun `certificate for another hostname says the address does not match`() {
+        assertEquals(
+            "The server's certificate doesn't match this address. Use the address the " +
+                "certificate was issued for",
+            SSLPeerUnverifiedException("Hostname 10.0.0.5 not verified").toUserMessage(),
+        )
+    }
+
+    @Test
+    fun `a handshake failure with no certificate cause stays a network error`() {
+        assertEquals(
+            IOException("x").toUserMessage(),
+            SSLHandshakeException("Connection closed by peer").toUserMessage(),
+        )
+    }
+
     @Test
     fun `unmapped exception falls back to default`() {
         assertEquals(