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

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> {
-    if let Ok(resolver) = resolver_from_system_conf().await {
+pub async fn get_provider_by_mx(context: &Context, domain: &str) -> Option<&'static Provider> {
+    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());
-        assert!(get_provider_info("google.com", false).await.unwrap().id == "gmail");
+        let t = TestContext::new().await;
+        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(())
+    }
 }