fix missing MX resolver eg. on android (#2852)

* fix missing MX resolver eg. on android

switching completely to /etc/resolv.conf (see #2780)
does not work at least on some Androids
(see https://github.com/deltachat/deltachat-android/issues/2151 )

therefore, we use the old approach as a fallback.

* log a warning, when we again have problems with figuring out MX resolvers

deltachat-ffi/src/lib.rs

@@ -3753,7 +3753,11 @@ pub unsafe extern "C" fn dc_provider_new_from_email(
 
     match socks5_enabled {
         Ok(socks5_enabled) => {
-            match block_on(provider::get_provider_info(addr.as_str(), socks5_enabled)) {
+            match block_on(provider::get_provider_info(
+                ctx,
+                addr.as_str(),
+                socks5_enabled,
+            )) {
                 Some(provider) => provider,
                 None => ptr::null_mut(),
             }

examples/repl/cmdline.rs

@@ -1185,7 +1185,7 @@ pub async fn cmdline(context: Context, line: &str, chat_id: &mut ChatId) -> Resu
             let socks5_enabled = context
                 .get_config_bool(config::Config::Socks5Enabled)
                 .await?;
-            match provider::get_provider_info(arg1, socks5_enabled).await {
+            match provider::get_provider_info(&context, arg1, socks5_enabled).await {
                 Some(info) => {
                     println!("Information for provider belonging to {}:", arg1);
                     println!("status: {}", info.status as u32);

src/configure.rs

@@ -221,7 +221,9 @@ async fn configure(ctx: &Context, param: &mut LoginParam) -> Result<()> {
             "checking internal provider-info for offline autoconfig"
         );
 
-        if let Some(provider) = provider::get_provider_info(&param_domain, socks5_enabled).await {
+        if let Some(provider) =
+            provider::get_provider_info(ctx, &param_domain, socks5_enabled).await
+        {
             param.provider = Some(provider);
             match provider.status {
                 provider::Status::Ok | provider::Status::Preparation => {

src/oauth2.rs

@@ -58,7 +58,7 @@ pub async fn dc_get_oauth2_url(
     redirect_uri: &str,
 ) -> Result<Option<String>> {
     let socks5_enabled = context.get_config_bool(Config::Socks5Enabled).await?;
-    if let Some(oauth2) = Oauth2::from_address(addr, socks5_enabled).await {
+    if let Some(oauth2) = Oauth2::from_address(context, addr, socks5_enabled).await {
         context
             .sql
             .set_raw_config("oauth2_pending_redirect_uri", Some(redirect_uri))
@@ -79,7 +79,7 @@ pub async fn dc_get_oauth2_access_token(
     regenerate: bool,
 ) -> Result<Option<String>> {
     let socks5_enabled = context.get_config_bool(Config::Socks5Enabled).await?;
-    if let Some(oauth2) = Oauth2::from_address(addr, socks5_enabled).await {
+    if let Some(oauth2) = Oauth2::from_address(context, addr, socks5_enabled).await {
         let lock = context.oauth2_mutex.lock().await;
 
         // read generated token
@@ -225,7 +225,7 @@ pub async fn dc_get_oauth2_addr(
     code: &str,
 ) -> Result<Option<String>> {
     let socks5_enabled = context.get_config_bool(Config::Socks5Enabled).await?;
-    let oauth2 = match Oauth2::from_address(addr, socks5_enabled).await {
+    let oauth2 = match Oauth2::from_address(context, addr, socks5_enabled).await {
         Some(o) => o,
         None => return Ok(None),
     };
@@ -253,13 +253,13 @@ pub async fn dc_get_oauth2_addr(
 }
 
 impl Oauth2 {
-    async fn from_address(addr: &str, skip_mx: bool) -> Option<Self> {
+    async fn from_address(context: &Context, addr: &str, skip_mx: bool) -> Option<Self> {
         let addr_normalized = normalize_addr(addr);
         if let Some(domain) = addr_normalized
             .find('@')
             .map(|index| addr_normalized.split_at(index + 1).1)
         {
-            if let Some(oauth2_authorizer) = provider::get_provider_info(domain, skip_mx)
+            if let Some(oauth2_authorizer) = provider::get_provider_info(context, domain, skip_mx)
                 .await
                 .and_then(|provider| provider.oauth2_authorizer.as_ref())
             {
@@ -356,30 +356,33 @@ mod tests {
 
     #[async_std::test]
     async fn test_oauth_from_address() {
+        let t = TestContext::new().await;
         assert_eq!(
-            Oauth2::from_address("hello@gmail.com", false).await,
+            Oauth2::from_address(&t, "hello@gmail.com", false).await,
             Some(OAUTH2_GMAIL)
         );
         assert_eq!(
-            Oauth2::from_address("hello@googlemail.com", false).await,
+            Oauth2::from_address(&t, "hello@googlemail.com", false).await,
             Some(OAUTH2_GMAIL)
         );
         assert_eq!(
-            Oauth2::from_address("hello@yandex.com", false).await,
+            Oauth2::from_address(&t, "hello@yandex.com", false).await,
             Some(OAUTH2_YANDEX)
         );
         assert_eq!(
-            Oauth2::from_address("hello@yandex.ru", false).await,
+            Oauth2::from_address(&t, "hello@yandex.ru", false).await,
             Some(OAUTH2_YANDEX)
         );
-
+        assert_eq!(Oauth2::from_address(&t, "hello@web.de", false).await, None);
-        assert_eq!(Oauth2::from_address("hello@web.de", false).await, None);
     }
 
     #[async_std::test]
     async fn test_oauth_from_mx() {
+        // TODO: this does not test MX lookup, google.com is in our provider-db
+        // does anyone know a "good" Google Workspace (former G Suite) domain we can use for testing?
+        let t = TestContext::new().await;
         assert_eq!(
-            Oauth2::from_address("hello@google.com", false).await,
+            Oauth2::from_address(&t, "hello@google.com", false).await,
             Some(OAUTH2_GMAIL)
         );
     }

src/provider.rs

@@ -3,8 +3,11 @@
 mod data;
 
 use crate::config::Config;
+use crate::context::Context;
 use crate::provider::data::{PROVIDER_DATA, PROVIDER_IDS, PROVIDER_UPDATED};
-use async_std_resolver::resolver_from_system_conf;
+use async_std_resolver::{
+    config, resolver, resolver_from_system_conf, AsyncStdResolver, ResolveError,
+};
 use chrono::{NaiveDateTime, NaiveTime};
 
 #[derive(Debug, Display, Copy, Clone, PartialEq, FromPrimitive, ToPrimitive)]
@@ -81,6 +84,22 @@ pub struct Provider {
     pub oauth2_authorizer: Option<Oauth2Authorizer>,
 }
 
+/// Get resolver to query MX records.
+///
+/// We first try resolver_from_system_conf() which reads the system's resolver from `/etc/resolv.conf`.
+/// This does not work at least on some Androids, therefore we use use ResolverConfig::default()
+/// which default eg. to google's 8.8.8.8 or 8.8.4.4 as a fallback.
+async fn get_resolver() -> Result<AsyncStdResolver, ResolveError> {
+    if let Ok(resolver) = resolver_from_system_conf().await {
+        return Ok(resolver);
+    }
+    resolver(
+        config::ResolverConfig::default(),
+        config::ResolverOpts::default(),
+    )
+    .await
+}
+
 /// Returns provider for the given domain.
 ///
 /// This function looks up domain in offline database first. If not
@@ -89,7 +108,11 @@ pub struct Provider {
 ///
 /// For compatibility, email address can be passed to this function
 /// instead of the domain.
-pub async fn get_provider_info(domain: &str, skip_mx: bool) -> Option<&'static Provider> {
+pub async fn get_provider_info(
+    context: &Context,
+    domain: &str,
+    skip_mx: bool,
+) -> Option<&'static Provider> {
     let domain = domain.rsplitn(2, '@').next()?;
 
     if let Some(provider) = get_provider_by_domain(domain) {
@@ -97,7 +120,7 @@ pub async fn get_provider_info(domain: &str, skip_mx: bool) -> Option<&'static P
     }
 
     if !skip_mx {
-        if let Some(provider) = get_provider_by_mx(domain).await {
+        if let Some(provider) = get_provider_by_mx(context, domain).await {
             return Some(provider);
         }
     }
@@ -117,8 +140,8 @@ pub fn get_provider_by_domain(domain: &str) -> Option<&'static Provider> {
 /// Finds a provider based on MX record for the given domain.
 ///
 /// For security reasons, only Gmail can be configured this way.
-pub async fn get_provider_by_mx(domain: &str) -> Option<&'static Provider> {
+pub async fn get_provider_by_mx(context: &Context, domain: &str) -> Option<&'static Provider> {
-    if let Ok(resolver) = resolver_from_system_conf().await {
+    if let Ok(resolver) = get_resolver().await {
         let mut fqdn: String = domain.to_string();
         if !fqdn.ends_with('.') {
             fqdn.push('.');
@@ -143,6 +166,8 @@ pub async fn get_provider_by_mx(domain: &str) -> Option<&'static Provider> {
                 }
             }
         }
+    } else {
+        warn!(context, "cannot get a resolver to check MX records.");
     }
 
     None
@@ -169,6 +194,8 @@ mod tests {
 
     use super::*;
     use crate::dc_tools::time;
+    use crate::test_utils::TestContext;
+    use anyhow::Result;
     use chrono::NaiveDate;
 
     #[test]
@@ -218,12 +245,13 @@ mod tests {
 
     #[async_std::test]
     async fn test_get_provider_info() {
-        assert!(get_provider_info("", false).await.is_none());
+        let t = TestContext::new().await;
-        assert!(get_provider_info("google.com", false).await.unwrap().id == "gmail");
+        assert!(get_provider_info(&t, "", false).await.is_none());
+        assert!(get_provider_info(&t, "google.com", false).await.unwrap().id == "gmail");
 
         // get_provider_info() accepts email addresses for backwards compatibility
         assert!(
-            get_provider_info("example@google.com", false)
+            get_provider_info(&t, "example@google.com", false)
                 .await
                 .unwrap()
                 .id
@@ -242,4 +270,10 @@ mod tests {
         assert!(get_provider_update_timestamp() <= time());
         assert!(get_provider_update_timestamp() > timestamp_past);
     }
+
+    #[async_std::test]
+    async fn test_get_resolver() -> Result<()> {
+        assert!(get_resolver().await.is_ok());
+        Ok(())
+    }
 }