libstore: exponential backoff for downloads

Closes #932

`connect-timeout` gets replaced by an exponential backoff for the
download timeout where the initial value is controlled by the setting
`initial-connect-timeout`.

Per iteration, the upper limit of the timeout is increased set to

    timeout := min(max_connect_timeout, initial_connect_timeout * 2^i)

I decided to move the entire timeout / tracking of attempts into its own
class to not make the filetransfer implementation more complex. Also,
that allows us to write unit-tests for it.

Setting `--download-attempts` to `0` is forbidden now and an exception
will be thrown. For `--offline` we set it to `1`, the behavior is
equivalent to what it was before: whether the max tries were exceeded is
only checked after the first download exception got thrown, i.e. there's
still one attempt being made.

The end-result - with timeouts being caused by a wrongly set proxy -
looks like this:

    $ env HTTPS_PROXY=1.1.1.1 nix store ping --store https://example.com
    warning: error: unable to download 'https://example.com/nix-cache-info': Connection timed out after 5006 milliseconds (curl error code=28); retrying in 422ms ms (attempt 1/5)
    warning: error: unable to download 'https://example.com/nix-cache-info': Connection timed out after 10010 milliseconds (curl error code=28); retrying in 1003ms ms (attempt 2/5)
    warning: error: unable to download 'https://example.com/nix-cache-info': Connection timed out after 20020 milliseconds (curl error code=28); retrying in 2018ms ms (attempt 3/5)
    warning: error: unable to download 'https://example.com/nix-cache-info': Connection timed out after 40007 milliseconds (curl error code=28); retrying in 4087ms ms (attempt 4/5)
    error: unable to download 'https://example.com/nix-cache-info': Connection timed out after 80074 milliseconds (curl error code=28)

Change-Id: I9e8d08d78275bcf60080d663febc9e075243d36b

tests/unit/libutil/backoff.cc

@@ -0,0 +1,52 @@
+#include <gtest/gtest.h>
+#include <lix/libutil/backoff.hh>
+
+namespace nix {
+TEST(Backoff, defaults)
+{
+    auto initial = 5;
+    auto backoff = backoffTimeouts(
+        5, std::chrono::seconds(300), std::chrono::seconds(initial), std::chrono::milliseconds(1000)
+    );
+
+    BackoffTiming timings = *backoff.next();
+    ASSERT_EQ(10000, timings.downloadTimeout.count());
+    ASSERT_LE(1500, timings.waitTime.count());
+    ASSERT_GE(2500, timings.waitTime.count());
+
+    timings = *backoff.next();
+    ASSERT_EQ(20000, timings.downloadTimeout.count());
+    ASSERT_LE(3500, timings.waitTime.count());
+    ASSERT_GE(4500, timings.waitTime.count());
+
+    ASSERT_TRUE(backoff.next().has_value());
+
+    timings = *backoff.next();
+    ASSERT_EQ(80000, timings.downloadTimeout.count());
+    ASSERT_LE(15500, timings.waitTime.count());
+    ASSERT_GE(16500, timings.waitTime.count());
+
+    ASSERT_FALSE(backoff.next().has_value());
+}
+
+TEST(Backoff, capped)
+{
+    auto initial = 10;
+    auto upper = 300;
+    auto backoff = backoffTimeouts(
+        7,
+        std::chrono::seconds(upper),
+        std::chrono::seconds(initial),
+        std::chrono::milliseconds(1000)
+    );
+
+    BackoffTiming timings = *backoff.next();
+    *backoff.next();
+    *backoff.next();
+    *backoff.next();
+    *backoff.next();
+    timings = *backoff.next();
+    ASSERT_EQ(300000, timings.downloadTimeout.count());
+    ASSERT_FALSE(backoff.next().has_value());
+}
+}

tests/unit/meson.build

@@ -47,6 +47,7 @@ libutil_tests_sources = files(
   'libutil/async-collect.cc',
   'libutil/async-io.cc',
   'libutil/async-semaphore.cc',
+  'libutil/backoff.cc',
   'libutil/canon-path.cc',
   'libutil/checked-arithmetic.cc',
   'libutil/chunked-vector.cc',