Use resolv.conf to find name servers on Posix (#1394)

* Use resolv.conf to find name servers on Posix

* Fix doc comment

* Fix Windows

* Robustify resolv.conf parser

* Feedback

lib/net/modules/dns.toit

@@ -41,7 +41,7 @@ dns_lookup -> net.IpAddress
     if not server:
       client = default_client
     else:
-      client = ALL_CLIENTS_.get server --init=:
+      client = AUTO_CREATED_CLIENTS_.get server --init=:
         DnsClient [server]
   return client.get host --accept_ipv4=accept_ipv4 --accept_ipv6=accept_ipv6 --timeout=timeout
 
@@ -53,11 +53,64 @@ DEFAULT_CLIENT ::= DnsClient [
 ]
 
 // A map from IP addresses (in string form) to DNS clients.
-// If a DNS client has multiple servers it can query then they
-// are separated by slash (/) in the key.
-ALL_CLIENTS_ ::= {DEFAULT_CLIENT.servers_.join "/": DEFAULT_CLIENT}
+AUTO_CREATED_CLIENTS_ ::= {:}
 
-default_client := DEFAULT_CLIENT
+user_set_client_/DnsClient? := null
+dhcp_client_/DnsClient? := null
+
+RESOLV_CONF_ ::= "/etc/resolv.conf"
+
+/**
+On Unix systems the default client is one that keeps an eye on changes in
+  /etc/resolv.conf.
+On FreeRTOS systems the default client is set by DHCP.
+On Windows we currently default to using Google and Cloudflare DNS servers.
+On all platforms you can set a custom default client with the
+  $(default_client= client) setter.
+*/
+default_client -> DnsClient:
+  if user_set_client_: return user_set_client_
+  if platform == PLATFORM_FREERTOS and dhcp_client_: return dhcp_client_
+  if platform == PLATFORM_LINUX or platform == PLATFORM_MACOS: return etc_resolv_client_
+  return DEFAULT_CLIENT
+
+default_client= client/DnsClient? -> none:
+  user_set_client_ = client
+
+current_etc_resolv_client_/DnsClient? := null
+etc_resolv_update_time_/Time? := null
+
+etc_resolv_client_ -> DnsClient:
+  catch --trace:
+    resolv_conf_stat := stat_ RESOLV_CONF_ true
+    etc_stat := stat_ "/etc" true
+    resolv_conf_time := Time.epoch --ns=resolv_conf_stat[ST_MTIME_]
+    etc_time := Time.epoch --ns=etc_stat[ST_MTIME_]
+    modification_time := resolv_conf_time > etc_time ? resolv_conf_time : etc_time
+    if etc_resolv_update_time_ == null or etc_resolv_update_time_ < modification_time:
+      etc_resolv_update_time_ = modification_time
+      // Create a new client from resolv.conf.
+      resolv_conf := (read_file_content_posix_ RESOLV_CONF_ resolv_conf_stat[ST_SIZE_]).to_string
+      nameservers := []
+      resolv_conf.split "\n": | line |
+        hash := line.index_of "#"
+        if hash >= 0: line = line[..hash]
+        line = line.trim
+        if line.starts_with "nameserver ":
+          server := line[11..].trim
+          if net.IpAddress.is_valid server --accept_ipv4=true --accept_ipv6=false:
+            nameservers.add server
+      current_etc_resolv_client_ = DnsClient nameservers
+  return current_etc_resolv_client_ or DEFAULT_CLIENT
+
+ST_SIZE_ ::= 7
+ST_MTIME_ ::= 9
+
+stat_ name/string follow_links/bool -> List?:
+  #primitive.file.stat
+
+read_file_content_posix_ filename/string size/int -> ByteArray:
+  #primitive.file.read_file_content_posix
 
 CLASS_INTERNET ::= 1
 

src/compiler/propagation/type_primitive_file.cc

@@ -38,6 +38,7 @@ TYPE_PRIMITIVE_ANY(mkdtemp)
 TYPE_PRIMITIVE_ANY(is_open_file)
 TYPE_PRIMITIVE_ANY(realpath)
 TYPE_PRIMITIVE_ANY(cwd)
+TYPE_PRIMITIVE_BYTE_ARRAY(read_file_content_posix)
 
 }  // namespace toit::compiler
 }  // namespace toit

src/primitive.h

@@ -610,6 +610,7 @@ namespace toit {
   PRIMITIVE(is_open_file, 1)                 \
   PRIMITIVE(realpath, 1)                     \
   PRIMITIVE(cwd, 0)                          \
+  PRIMITIVE(read_file_content_posix, 2)      \
 
 #define MODULE_PIPE(PRIMITIVE)               \
   PRIMITIVE(init, 0)                         \

src/primitive_file_posix.cc

@@ -89,6 +89,32 @@ Object* return_open_error(Process* process, int err) {
   OTHER_ERROR;
 }
 
+PRIMITIVE(read_file_content_posix) {
+#ifndef TOIT_POSIX
+  UNIMPLEMENTED_PRIMITIVE;
+#else
+  ARGS(cstring, filename, int, file_size);
+  ByteArray* result = process->allocate_byte_array(file_size);
+  if (result == null) ALLOCATION_FAILED;
+  ByteArray::Bytes result_bytes(result);
+  int fd = open(filename, O_RDONLY);
+  if (fd == -1) return return_open_error(process, errno);
+  int position = 0;
+  for (position = 0; position < file_size; ) {
+    int n = read(fd, result_bytes.address() + position, file_size - position);
+    if (n == -1) {
+      if (errno == EINTR) continue;
+      close(fd);
+      OTHER_ERROR;
+    }
+    if (n == 0) INVALID_ARGUMENT;  // File changed size?
+    position += n;
+  }
+  close(fd);
+  return result;
+#endif
+}
+
 // Coordinate with utils.toit.
 static const int FILE_RDONLY = 1;
 static const int FILE_WRONLY = 2;

src/primitive_file_win.cc

@@ -563,6 +563,11 @@ PRIMITIVE(cwd) {
   return result;
 }
 
+PRIMITIVE(read_file_content_posix) {
+  // This is currenly only used for /etc/resolv.conf.
+  UNIMPLEMENTED_PRIMITIVE;
+}
+
 }
 
 #endif  // TOIT_WINDOWS.