Don't queue a build when not enough memory is available

Example error:

```
2021/02/15 01:43:12 [INFO] docs_rs::docbuilder::rustwide_builder: building package regex 1.3.1
Error: Building documentation failed

Caused by:
    not enough memory to build regex 1.3.1: needed 3072 MiB, have 2909 MiB
help: set DOCSRS_DISABLE_MEMORY_LIMIT to force a build
```

Author: jyn514

src/config.rs

@@ -55,6 +55,7 @@ pub struct Config {
     pub(crate) toolchain: String,
     pub(crate) build_cpu_limit: Option<u32>,
     pub(crate) include_default_targets: bool,
+    pub(crate) disable_memory_limit: bool,
 }
 
 impl Config {
@@ -101,6 +102,7 @@ impl Config {
             toolchain: env("CRATESFYI_TOOLCHAIN", "nightly".to_string())?,
             build_cpu_limit: maybe_env("DOCS_RS_BUILD_CPU_LIMIT")?,
             include_default_targets: env("DOCSRS_INCLUDE_DEFAULT_TARGETS", true)?,
+            disable_memory_limit: env("DOCSRS_DISABLE_MEMORY_LIMIT", false)?,
         })
     }
 }

src/docbuilder/rustwide_builder.rs

@@ -11,6 +11,7 @@ use crate::utils::{copy_doc_dir, parse_rustc_version, CargoMetadata, GithubUpdat
 use crate::{db::blacklist::is_blacklisted, utils::MetadataPackage};
 use crate::{Config, Context, Index, Metrics, Storage};
 use docsrs_metadata::{Metadata, DEFAULT_TARGETS, HOST_TARGET};
+use failure::bail;
 use failure::ResultExt;
 use log::{debug, info, warn, LevelFilter};
 use postgres::Client;
@@ -309,6 +310,26 @@ impl RustwideBuilder {
         }
 
         let limits = Limits::for_crate(&mut conn, name)?;
+        #[cfg(target_os = "linux")]
+        if !self.config.disable_memory_limit {
+            let mem_info = procfs::Meminfo::new().context("failed to read /proc/meminfo")?;
+            let available = mem_info.mem_available.unwrap_or(
+                // Assume about half of cached memory can be reclaimed without thrashing.
+                // This is a terrible hack.
+                mem_info.mem_free + mem_info.cached / 2,
+            );
+            if limits.memory() as u64 > available {
+                bail!("not enough memory to build {} {}: needed {} MiB, have {} MiB\nhelp: set DOCSRS_DISABLE_MEMORY_LIMIT=true to force a build",
+                    name, version, limits.memory() / 1024 / 1024, available / 1024 / 1024
+                );
+            } else {
+                debug!(
+                    "had enough memory: {} MiB > {} MiB",
+                    limits.memory() / 1024 / 1024,
+                    available / 1024 / 1024
+                );
+            }
+        }
 
         let mut build_dir = self.workspace.build_dir(&format!("{}-{}", name, version));
         build_dir.purge()?;